Mono is sent to the centre channel because, being only one channel, it would be weird if it sent it to the left, or to the right. But in a theatrical setting, you aren't hearing it from one speaker at the front of the house. It is playing through many speakers so as to encompass the full room with front-facing sound. This is why, in a home theatre setting, it is appropriate to send it to all your forward-facing speakers, but namely the left and right which are accoustically positioned to fill your listening space. I suppose it depends on room size to a degree. But again, in any sort of concert or theatre environment, there is no single centre speaker that provides audio for the entire room. Centre-default plays to the reality of one-channel audio, in that it is the most appropriate position if one speaker represents one channel, but it betrays the ideal listening experience. Your left and right speakers will be better manufactured as well, even if your centre channel is of high quality--the left and right are always the best speakers in the set-up, and the ones designed for the widest and furthest sound dispersal. Which is why in mono theatres, you hear the soundtrack from the left and right throughout multiple speakers, to best fill the room.

That's my two cents, anyway. Mono 2.0, as I take it, is a way to "cheat" the default of only ending up with a centre speaker playback (or, in simpler stereo set-ups, of only having a right speaker playback) and ensuring that you have the fullest playback sound.