I've taken out my $1200 Sigma 70-200 2.8 EX on the field, and the concern has crossed my mind.
I always have a B+W MRC UV(0) filter on it for protection (which is worth a fair amount in itself), so I usually add a second, cheapo Optex UV filter over it to protect against BBs; the Optex filter is only worth about $40 so I don't mind it getting damaged, and in the case of a hit it should rob the BB of most of its energy. Failing that, or in the case of multiple BBs, there's the second (usual) UV filter in place.
I also recommend using your lens hood, which should help deflect shots coming from "off camera" (the logic being if you're looking through the viewfinder and you see someone pointing a gun in your direction, you'll probably be smart enough to point the camera down -- but outside your field of view you may not see the person about to shoot in your direction)
For daylight shots, the cheaper filter won't affect things that much as long you aren't shooting into a light source (cheap, uncoated filters will flare/reflect light, especially with dSLRs).
But in the end, it comes down to calculated risk. If you aren't willing to gamble the lens, don't take it on the field.