I feel the need to have something to "hold the diaper in place". If i have, like, baggy pants, the diaper can just sag freely. That feels sorta weird. So i'll wear additional underwear in that case.
I prefer a onesie with a crotch snap to achieve the same effect of added support. I guess that doesn't count as additional "underwear"? In most cases, this is what i'll wear instead of additional underwear.
So, yes, i definitely usually wear something on top of a diaper. But it's not always underwear, per se?