There are a multitude of reasons to use a Drum Shield.  However, it is generally considered to be good form to apply all Drum Shield rules equally.  Here’s a stage where this rule was not followed.  One can only imagine the animosity this treatment would create between the two drummers.  Although, anyone who likes drummers enough to have two on stage is probably a drummer himself, so it’s safe to assume that the drummer who’s kit is stage left is the band leader.

Drum Shield Discrimination

Easy to tell which drummer is considered safe.

This image and a great write up on drum shielding can be found here.