Seriously, it's great – it would have been perfect for my track "Atmospheric Oppression by   caerphoto, but even beyond somewhat gimmicky 'weather' effects, it can be used to alter the mood of a track significantly. Here's a regular 'sunrise' screenshot, from the finish line of one of my in-progress tracks:



Kinda ... dull? Here it is with some Ambience MT adjustments:



Much prettier!

This sort of thing could technically be achieved with the usual in-race MT but it's quite fiddly, and you'd have to copy it for the intro and replay, so it's not nearly as easy to maintain, and if you want to do anything like ambience that changes as the race goes on, it gets much more complicated.

So yay Ambience MT – so many possibilities