Ours are graded, so there are programs out there that do it. I can't think of any good justifications against the morality of "should be P/F", except to say that grades matter *so* little in determining future success in academia that the standard is to make everyone look good on paper - in most research-focused programs even the actual classes are this way. So if you're planning in advance to apply professional school where grades matter, you can give yourself a little bit of an unfair advantage if you choose carefully. But again, this is a workable GPA-booster only if vet schools actually average ugrad and grad GPA.
I agree with the above that there seems to be a push toward "veterinary scientists" so even if you don't get the GPA boost, having (published) research experience might make you interesting enough to overcome a less-than-stellar GPA (but maybe not enough to overcome a truly awful GPA, seems like a lot of schools have minimum cutoffs). It will also show you're more mature, dedicated, worldly, blah, blah...