Some of us are more dexterous than others (or sinister if
we use left hand mice?).  If you can handle a 6 button mouse
with a scroll wheel attached (or a one handed keyboard  for
that matter), more power to you.  I just don't do well
with more than 3 and want to make sure they don't all
disappear.  For that matter, I can just barely handle chording on
acme and I know lots of old mac users that were apalled at
3 button mice.

I had a mouse with 4 for a while and ended up sticking a straw
under the 4th one so I wouldn't keep accidentally clicking it.
The mice with wheels doubling as a button 2, I find even more
annoying since it makes button 2 harder to use than the other
two buttons.  I tend to hit it by accident.