i pretty much always layer my drums. mainly because its rare that i ever hear a single drumsound and feel its complete enough. i might like a kick, thats deep and boomy, but it might not have enough initial punch, so i'll put a punchy one with it, and have a beefier kick sound.
same with snares, i might like the back end of a snare, like the "air" reverberating after the initial smack, but i might not like the smack, or i might want it to smack harder, or vice versa, i might have one with a dope smack, but no tail end. so i'll put two or more together for the desired thickness and sound
for instance lets look at a classic premier beat
the drums he took from this track (skip to 5:28)
premier sampled one shots and the quick, snare-kick-snare, for the little change in his pattern. those drums are much busier than the ny state of mind drums, but they are the same sounds. the difference is preme layered a kick over the kick to give it more of a thump, the punch was already relevant in that sample. with the snare he gave it more of a mid to low smack, while preserving the high crack
now, he might not have layered drums over it, he very well could have just eq'ed and mixed them to beef them up. i've just read in interviews with him he likes to layer stock sounds (from mpc's and sp1200's etc, not stock computer sounds) with his sampled drums.
so im guessing he just put an mpc kick and snare in with them and mixed it a little for desired effect.
either way word. you just kind of layer to make it sweet.
now alot of drums you'll find in kits are already layered sounds, but they are layered how whoever made them wants them. you don't need to layer sounds. i just like to it makes it more custom and shit