Hmm, I learn something new every day.

I figured maybe there was some technical reason they had to use
vacuum, since most of the instruments one can buy seem to be either
electrical or vacuum. From your answer, I suppose there is no real
reason, maybe most of the non electrical ones are vacuum because that
is just the way things are.
Colibri's response is why most are driven off the vacuum side but it isn't a
requirement. Like was mentioned the filter provides some protection.
According to an article in the newest Private Pilot magazine on twins. The
Beech Duchess Model 76 that they review is run off the pressure side of the
pumps (page 52, February 2005). They also mention most Beechcrafts are done
that way.
John