You are absolutley right, you can't tell what someone has growing in their intestinal track, however I can tell you that as far as MRSA and VRE, from a clinical standpoint they can be transmitted from eating feces.

What many people do not understand is that the medical community are seeing many ways in which 'bugs' are able to enter the body in ways that were not known before. Its a fact. Everytime we have a patient in the hospital with MRSA it's cultured, guess what? 9 out of 10 times it shows up in feces first.
Not to mention that it means isolation precautions; the reason for that is because it is highly contagious.

I won't even go into HBV or HBC, but the point is, practicing this activity is as *RISKY*.