I don't know if certain songs will be able to be the same with one person.
Like if Danketsu makes it in the game I don't know how that would be one person.
Yeah, that's the only bad point with it

Like the earlier point that some songs may look awkward with only 4 people XD
Though, I know that (at least) The World is All One could hold itself pretty well as a solo (odd...) dance-wise.
bonus controllable train during performances option?
With the rising popularity of the train, there's no way that'd be free

Though I secretly hope The tR@INMASTER isn't going to be released in February 2011 XD