I completely dissagree that "predators" are main buyers and spenders of FSP.If you are a non-PvP player,the best possible equipment can help you to level faster.On the other hand,you can be a succesful PvP player with moderate gear.Bottom line,I don't think there is much difference in spending fsp between PvP and non-PvP players
Allow me to disagree too (at least for the levels I know).
For hunting/leveling purposes, high cost legendary sets are irrelevant. The logic behind gearing for hunting is having the exact stats that allow you to do 1 stamina kills of the particular critter you are hunting. These stats vary a lot and a small (2-3 items) set gives you more flexibility).
Say the foe you are going to hunt has a huge possible def - you need to boost your attack through gear. Depending on your protective strategy, you can have either armor or defence at just the needed level to avoid getting killed (too often, if you use defence :) ). Your damage needs to be above your targets armor + HP...etc. Since your targets vary a lot, you will need to vary your setup too to be perfectl effective and large sets, however good they look, are not a good long term option.
Take the carnage set - it has a risible attack rate that literally cripples your leveling/hunting potential against any creature with more then 80-100 defence. The Nightshade set has puny damage (making one stam kills a lot more difficult)...etc.
Smaller sets are the best for they allow you to customize your gear to the enemy you are facing. Small sets that give bonuses to attack /damage are the best hunting wise (Girath, Zhann, Khale, Kragh etc) Small legendary sets (Plague, Deep, Mancrusha) are of course great for hunting/leveling - however they can be replaced effectively by a custom setup for 1% of the cost.
I think Delux discussed the usefulness of sets for leveling in an older thread, reaching the same conclusion.
I went infernus - giggles (till lvl 43!) - flamed till now. No other full sets, no piece of gear worth more then 4-5 FSP. 1.5-2 levels per day. I plan to switch to Khale and maybe Darklore later. The sets are there mostly for elite hunter (priceless mod).
PvP is different. You want to have better stats then people at your level (as opposed to the critters in PvE). For this, legendary sets (Frost dragon, Fire Dragon, Plague etc) are a must. Sure you can PvP with non legendary sets, however you will be killed more often by bounties, and targeted by other PvP players at your level (being bountied by a +30 levels is hard to avoid - however, being PvP ed by a +5 levels can be). And an expensive legendary set is the best way to say "I will bounty you - look how rich I am" :)