I would imagine the origins were very much to do with solving an engineering problem. Look at the early monoplanes, they have some very interesting posts with lots of string and bracing to keep the wing in place..
To more recent examples, the major downside is that the biplane configuration adds heaps of drag. Generally the monoplanes have a larger wing & greater wingspan to make up for only having a single plank - I guess that gives a better lever, but also more roll inertia. I don't know about the intrinsic roll rate comparison; but can confirm that in practical terms, a pitts rolls pretty damn fast, as does an extra
The thing that strikes most if you ever stand next to a pitts is how small it is, and how tiny the wingspan is; if I put my fingers against the side of the fus, I can almost touch the wingtip with the other arm - and thats the 2 hole variety.