I recently saw scarra and ReginaId playing Twisted Fate and getting Kage's lucky pick instead of double dorans. Doesn't it make more sense for most builds to get Kage's, save about 200 gold, and get money faster for shoes/mid-game items?

Kage's (765g): 25AP, gp10
Doran x2 (950g): 30AP, 160HP, 10mregen

This build does tend to favour TF since he doesn't really need mana regen. I'm wondering how effective this might be on other mid champions.

Well, 160 HP is quite a lot, considering most AP mids have about 700 HP at level 5. Because TF isn't exactly an AP caster, he has about 800 HP at 5, but in either case, 160 HP is pretty good. He's also got a tad less armor, like 27 vs 30.

Going GP5 or Kage's means you know the lane will be a passive farmfest so you want to build up gold for late game. It eventually builds into DFG, so it won't be wasted. Dorans provides the ability to be tankier mid lane and to stay in lane longer VIA mana regen. It also helps the mid laner catch up if behind due to the gold value for the item. You can build Kage's on any lane that you think will end up a farm fest without many kills.