To be honest I would hazard a guess that most people have a favourite "start" terrain for all maps, and that would either be meadow of some sort, water or plains. I can't imagine that most people would want to choose different start terrains each time.
Not really. So far, I built 6 maps, 3 of which i haven't uploaded yet. And I always used different base terrain. Respectively, one type of desert, another type of desert, mountain meadow, winter water, desert water 2x (but if I could rebuild one map I'd make it summer water). In 3 of the maps I didn't change all the map to the base terrain because there wasn't such a high prevalence, but it could have saved a bit of time. Anyway I don't see why the program should restrict my choice, especially when it doesn't seem any easier. I suppose letting one pick among all exhisting terrains should actually be easier than letting one pick among all terrains minus some.