In reply to Morts:
I always stayed away from them because they would be unlikely to fit sufficiently well.
Baselayers should be skin-tight to wick properly. In Icebreaker I take an XXL top and L bottoms, and onesies are seldom sized with such a difference. What fits on top is too loose on the bottom, or vice versa.
Also, how do you crap? Zips on onepiece suits were always a problem, and would be uncomfortable on a baselayer, and would have to match the mid and outer layers to avoid any accidents.
Balancing all these aspects is maybe possible if you have a very average body shape, lots of time to compare models and lots of money to get the right one, but I just can't see they're worth the effort and expense. A properly fitting top and bottom are more versatile and so better value for money.