Didn't Sega teach us that add-ons are a bad idea, partly because they split the userbase? Adding anything significant to a system halfway through its market cycle just isn't a good idea. The N64 Expansion Pak and Wii Motion Plus are essentially just accessories, since only a handful of games actually require them. The Kinect and PSMove are a little more successful, but still aren't the new standard for those systems and haven't really gotten out of accessory status.
The only exception I can think of is the PlayStation's Dual Analogue and then Dual Shock controllers, but they were added before the system gained significant market share. There were also still few games that required them, so for people who bought one of the first systems they were not a necessity. This is also why the 3DS slide pad add-on may also work out, since it's happening early on and likely won't be required for just about everything.