I'm supporting this for the sake of knowing that more keys would make it easier. Coming from Stepmania to osu!, having the option of two layouts of keys does make it much easier. But since we have one hand on a mouse, and one hand on the keyboard, having 3, or even 4 keys to stream with would make it easier for most.
For those that don't think that to be the case, place your hand on the desk or on the keyboard, and tap with your tapping fingers as fast as possible. Now do it with four fingers, and go pinky finger to index. If you can, you should notice you're taping four times out of sync, but going from pinky to index you're tapping faster then just your two tapping fingers. Now if you were able to learn to do the lot of it in sync? Well... you theoretically could stream twice as fast. But physically you'd end up being roughly 50% faster.
I use both keyboard and mouse, but I know of people that could easily benefit from this, both tablet and mouse users.
Also, implementing the possibility for it with taiko would make the pro's OP. Just sayin'. +1 from me.