The "jump in at any time with Boost Mode" excuse is valid, but still shaky. Doesn't hurt to give the player options. Nintendo shouldn't assume that people have enough Wiimotes to play 5-person multiplayer. Not everyone wants to play in boost mode, so if 2, 3, or 4 players want to play as characters on-screen, they should be able to do so with the Gamepad.
This issue has shown to be detrimental to some peoples' enjoyment of the game, so Nintendo should take note if they can. Could a patch help out? Hopefully Nintendo is on it, but even then, a patch at this point won't make up for the fact that some holiday gatherings may have been slightly soured when folks realized they can't just let someone use a Gamepad for regular play. A little foresight would certainly help in the future. This sort of thing seems so obvious.