Carries are, undoubtedly, an important part of this game. A carry needs to be baby-sitted in all stages of the game, a team setup usually can be built around a carry (protect the Kog). Such tactics exist because, not only in LoL but in all Mobas, carries were characters who required loads of farm and team commitment to succeed. They are an essential part of MOBA games. And still I often see people pledging to nerf AD carries because late game they one shot everyone...
There's a clear attempt to homogenize the DPS through out the game. Riot wants to avoid characters who explode after X minutes, and I for one do not understand. That's what a carry is supposed to do! Being **** tier early game, and then make up for it late game.
The upcoming Phantom Dancer nerf makes me wanna cry a little. The GD argument? They run too fast. Seriously, what the heck is wrong with you? Of course they run fast, they are the squishiest class in game and have a big target drawn on their head saying "Kill me or I kill your team".
Every bruiser/assassin has at least one gap closer, and while a good support and summoners spells can deal with them, mobility is and will always be an inherent part of being a Carry.
As a Vayne player I'm deeply concerned about what's going to happen to her. I can't think of a champion who depends on PD more than she does, especially considering that there are no other alternatives as far as getting Attack Speed goes. Yes, Riot is introducing a new item which gives 50% attack speed and bla bla bla, but that doesn't suit her because its meant for AoE carries, something Vayne is not.
I'm all up for item diversity, but not when that requires certain champions and even classes to be changed dramatically.
I hope Riot hasn't forgotten what made playing AD fun and seriously considers new items for pure Attack Damage carries such as Vayne or Draven who have no other abilities to compensate for the AttackSpeed and mobility lost.
Tl;Dr: S3 will mean AD carries will be a lot weaker and that doesn't make sense.