There is absolutely no reason for drums to be in stereo....or basslines for that matter.....or vocals!
Keep the stereo for special spacial effects and maybe fancy morphing lead lines.

I guess they made it so you can use the SP-404 as a field recorder as well as a drum machine / sampler.

I had the same problem. You need to upgrade the OS.

that depends on what music you are makeing!
if you are using your sp-404 to program drums for your rock music then you'll really need the drums to be stereo to sound good! but if you're programming hip hop beats then a mono drumset is perfect..

vocals do sound mutch better in stereo to if you f.ex. use ping pong delay or simply stereoreverb...

but mostly for hip hop stereo are only needed for thins like leads and fat pads..

I mean there is a big difference between stereo sound vs. mono even on drums. It does depend on the sound you are looking for but sometimes mono drums over stereo samples sound dry and not as full as the whole sound. as well as certain sounds need to be stereo to get the pan effect that comes with the sound. Mono strips your sounds of any of that. But if you need to save time or space do what you need to do.