To avoid unintentionally spreading viruses among Windows users, I guess. (though all of those ought to have virus programs of their own- still, even if it's picked up it won't make the Mac look good)
unless you pass on loads of attachments there is no point, the likelihood of a word document having a macro virus is miniscule if it comes from someone you know, you cant spread a virus on a mac unless you receive and send the thing yourself