Almost every game has level restrictions on gear and I believe it to be a good thing. The problem with the current system is that the penalty is not so great that it makes the item useless if misused. A rank 50 weapon that gives 200 attack, should NOT give 100 attack at level 1. Instead, at level 1, it should give 0 attack. Since it does not, people tend to abuse this and while, in the long run, it makes the player gimp, they still continue to use the item and then complain when it "breaks" after 3 fights. The same goes for mages running around in tank gear or tanks running around in cloth gear. Essentially, just like every other "problem" with this game, it wouldn't be a problem if the players weren't so ignorant.

tl;dr we need gear restrictions because the player base isn't smart enough to decide what gear works best for which job/level.