That was something that was really neat about ffxi. You aren't the only person that has mentioned wanting that.
I think it's a great idea too.

I'm sure it's on SE's list of stuff they want to try. I'd imagine it's one of the harder things to balance. Imagine you have a great sword that gives you a strong skill, but then suddenly another boss kill gives you a stronger sword, but it has a gimp ability.
So like, you'd have to decide to take items based on abilities AND their stats, etc.

People already get mad for people trying out weird combos of gear. Just think of all that drama this would cause.

"I'm using my lvl60 sword cause it has a skill I like. It doesn't matter I'm level 80 now. I'm white knight of justice, I will tank as I please" (Tank immediately dies to a lvl80 rabbit because of insufficient vitality given by poor choice of gear)