relevant to the fish and terrain (and or getting them in before the sharks get them) - would rather go lighter but in some circs lock and pull is necessary....

Comes down to if you are fishing for fun or to put food in the freezer.
Go hard first to get something in the icebox then swap to lighter gear, that way you are not going home empty handed

 I was once a very light tackle fisherman, way back,when everyone else here thought 30lb was light, and 50lb was stock for mackerel-- thought 4kg could do everything--then took stock of the number of fish sent away with gear/lures stuck in their mouths, all in the cause of my ego. Hearing of the 2kg/4kg fishjermen talking of losing 20 rapala magnums in the course of a comp--you've probably condemned these fish to a slow death, all for the sake of a line class capture. Or being a reluctant witness to an apparently endless fight of a not-particularly-interesting fish, just because the angler was on 4kg. F*ck.That. Sorry if I am offending anyone, but , after very long experience, I am not a fan of very large  fish on very light gear. The Ningaloo coast really sealed it for me, you just cannot go light, either because you are only giving the sharks a better go at your catch, or the most ordinary fish will brick you. and hence be left with the hook/hooks and a length of line trtailing from his mouth.So,up there, unless you are fishing over clean sandy bottom, and there are no sharks around, don't go under 30lb.(unless you are fishing for the small species) All our sails used to be caught on 20lb, which was fine, but, as our gear has to be dual purpose, 30lb or 50lb  is more commonly used. One thing about that--locking up on a sail on heavy (50Lb plus gear) seems to give a longer fight, as they cannot burn out on a mad rampage ealy in the fight--I like to let them run fast, tire quickly, then the release is quicker and cleaner, IMO. And this seems to apply to other species--the harder you pull, they harder they pull in return.

Anyway, back on topic--be flexible, it's all about doing what you enjoy, but remember that every fish you don't intend to kill should be released in as good a condition as possible, and an overly prolonged fight is reducing his chances of survival.

Have 'battled' a salmon for 50 minutes on a 2kg reel, and skull dragged a 5kg tuna in on 200lb line, I agree with an earlier comment. Get your feed, then play.

This year ive been taking out pe6 popping gear for salmon and its the most fun ive had catching salmon in a long time. Lock the drag right up and hold on tight, you can feel the actual power of fish rather than it pulling drag

 Bit of both. Line class fishing is my passion. Particularly light tackle billfish. 

Heavy stuff for bottom bashing when there are sharks around. My mono line class gear gets more use than my braid outfits though. 

Im with you beau, awesome fun.  You get to feel their real power in the hook up and they are green as when you land them.

  I like light line for small fish.

Funny one chasing pinks off the stonnes in kalbarri wear a 6 kg+ pink makes your 50 lb braid feel like 6 lb as they try and stitch u up every chance they get (Hey tangles!) but when I get out in the tub generly fish 20lb for them and this feels just right.

Another thing I found is when chasing big mulloway snaps and whatever else might turn up on the beach I consider 30lb light Now and run a minamim of 50lb and up to 80lb braid for them conciderin 2 experience Fisho called my last 25 kg mulloway for a shark after 25 mins of blistering runs on 50lb!

Spose the landbased guys don't get as much chance to fish real light but with a few more obsticles it makes it just as fun as using light gear in the boat

 like the comment about locking up and hanging on to salmon--some of these fish are really powerful, you just don't appreciate how much until you let them show it. Think about a 4kg spangled--the initial runs  are utterly awesome on 50lb if you are fishing vertically and trying to keep them out of the bottom in shallowish water. Call them for a much larger fish in the first moments, they'd pull an equivilant pinkie backwards. But I think the ultimate has to be the longnose--a 6 or 7 kg longnose emperor can peel line off anything. And we boat more big red bass since the gear was upped, including an utter horse of a weighed 10kg. Pinkies are a good light tackle adversary from a boat in the shallows--I always used 15lb mono with a small overhead, caugth a lot of good fish and had a good time, as they don't fight dirty. Then you would get bricked. Happened repeatedly one afternoon on a particular spot, so I went back a couple of days later with 20lb on a bigger outfit, and pulled up a little slimy cod, maybe 9kg, with a collection of our rigs in his mouth. Not long after, that bigger outfit saw me hooked up to something which went really hard, had me down to spool showing (300m of 20lb) and necessitated an anchor drop and  chase. Thought it may have been a really large sambo, the sun was just setting as a 42kg YFT popped up alongside the boat well exceeded the  australian line class record weight at the time. I'd never had got him on the Bantam 50 with 200m of 15lb loaded.

My catches of coral trout and big blue bone have increased here in Kalbarri since the gear has been heavier, tropical rules apply. And snapper pull just as hard, or harder, just not for so long.

And yeah, braid has made beach fishing sooo much better, beats fishing with a mono rubber band any day.

I enjoy light tackle sport fishing, more and more often i'm taking the 8kg out billfishing now i've learnt how to better handle the boat when chasing them down, i go even lighter chasing the queenies, trevs and tuna as theya re not likely to brick you. If i'm bottom bashing for a feed there is no sport if it's a good fish otherwise it becomes someone else dinner, if the sharks are thin that's nice to know and the smaller fish i'll take my time usually 50LB gear for all my bottom bashing rigs, i'll jig with 30lb but. 

Shallow water full of bommies and reef there is no 2-6kg line here, it's 20-30lb braid all the time.

my billfish gear is 10-15kg, i have 24kg gear but i've never taken it out, you knock the fish over in 10-20mins here anyway so i don't need a 40kg black going berko next to the boat after a 5 min fight, i like my safety and i'd rather spend 2 mins more reviving them to ahve them swim off strong and go back for another one.

Heavy and skull drag. Not much fun seeing your dinner disappear to the taxman. Or when I was in Dampier dont fish deep because the tax IMO was to much to pay. Ended up fishing 30m max after loosing some reds.

Otherwise as light as possible, because IMO it is a sport and the fish has to have a chance to "fight for its freedom". But then only whiting from shore stay in the bucket, all Tailor and Herring released. Fishing offshore tend to still use heavier gear and bring the fish in slowly after dragging it off the bottom. Prime species down here dont like Barotrauma much and dhu's and baldies suffer very badly, and despite release weights I dont think they survive well if skull dragged.

Like most people have said, it is more about the location and common sense.
I like to release a lot of fish and only keep the ones I want for a feed, but if a targeted fish wont release I then keep it to eat.
Going too light on some fish will see them exhausted and is probably not in their best interest if you want to let it go.
For Bream I have a few outfits ranging from 4 lb to 15 lb and use them in different locations, mostly going heavier in real snaggy situations. And yes I get busted of many times even with 15 lb braid and 20lb leaders.

 For me it's either light or heavy, not much in between (there are exceptions of course).

90% of my fishing is done with either 10lb or less or 50lb+

I do agree about different situations requiring different applications, and also with the above comments about locking up and feeling the raw power.