1. It is marketed as a three player game and the average consumer won't know the limitation?! The average consumer won't be sold by the internet at all, they will be sold because they played it at someone's house. The limitations will be apparent.
2. As a Wii owner this is nowhere near as bad a problem as the arbitrary limitation of one Balance Board. It has still been pretty successful.
Wii Fit has been quite successful, yes. The balance board itself, however, is barely supported by any other applications. A few games support it as a secondary, optional feature (Mario & Sonic, Punch Out), but no games outside of Wii Fit and Wii Fit Plus fully require the peripheral.