Well EAD alone employs about 1000 people and have developed everything from all types of racing games (inventing 2 of the most popular genres - kart and futuristic, they also pioneered motocross and jet ski), adventure games, platform games, strategy games, shooters, sports games, to whatever.
Then you got HAL Labs which makes great multiplayer based based games that are super addictive...
Game Freak, Game Designers Studio, and Camelot making RPG's
Intelligent Systems doing everything from strategy to action to RPG's
Silicon Knights doing mature adventure games
Retro Studios on action/adventure, although they could come up with pretty much anything by what it seems like...
NST on weird american racing/sports type games such as Wave Race and 1080
Zoonami working on some secret game....
And not really part of, owned by, or bound to Nintendo, but faithful to Nintendo, you got dudes like Factor 5, Sonic Team, and others....and Nintendo, although not believing in buying out companies (cough, didn't they buy Silicon Knights, Retro, and Intelligent Systems cough), they seem pretty content on handing a game over to any studio they like (from Amusement Vision to Namco) and publishing the title. So while they really only have 6 "first parties", or the dudes owned and operated by Nintendo or whatever you call them (Retro, NST, IntSys, HAL, EAD, Silicon Knights), they actually have a huge range of developers to choose from for any game they want to make. As long as Nintendo is publishing, what difference does it make?
I don't think they really need any genre-specific studios. Waste of time, money, and effort when you have so many teams to choose from...