Can someone please tell me what slow gem leveling adds to this game?

because they don't want everyone to farm the emerald weapon with fully maxed out knights of the round and Phoenix materia's :D

Gem levelling will improve in OB,Gems get 10% of your experience so therefore if your levelling faster in act 3 so will your gems.The slow down will happen later in your characters life.The knock on effect of that is you will also be able to accelerate gem levelling by using them on characters you've got into act 3.At the moment it's probably not worth the hassle to transfer gems when your highest effective level is lower.

Like some other people said, I think it feeds into the economy mostly. If you're a higher level, chances are you could probably trade for a decently high level gem. Maybe you don't have the patience to level a gem 20 levels or the cash to buy one right at your current state. You could trade for a gem for slightly cheaper that's only 5 levels below you.

I do agree that it may limit you to change around builds more freely, but GGG has said that while they want people to have the opportunity to make lots of different builds, they'd prefer you make an idea for a build and run with it. If it was really easy to change around skills and passives, you'd have less incentive to make more characters. You'd be more inclined to only make one duelist/witch/mara/etc and just change the skills/stats around.

And what else does it add? Another grind. Which is what these games are about. Well, that and loot. Gems are both so... Yeah.