Herbivores vs. Vegetarians

Okay, here's today's random thought: Are vegetarians the largest true herbivore ever to walk the earth? Sure, we have had lots and lots of really large herbivores that have walked the earth, but how many of the big ones really have a 100% vegetable diet? Let me put it to you this way: have you ever gotten a bug on your vegetables? Now think about how meticulously we wash them and look them over before eating them. Cows barely look before taking a mouthful. There is even a parasite whose life cycle depends on the fact that large herbivores don't look before eating. It invades an ant's body and takes over its mind, causing it to climb as high as it can on grass to hopefully be eaten by whatever is eating the grass.

So, what nutritional benefit do large herbivores get from the bugs they inevitably eat and, more importantly, is it a source of protein they rely on, or is it just incidental to their diet?