I'd say that for some people, it's fine. But if you think about it, the best milk is our own mother's milk, whatever creature we are -- and by the time we humans hit what once was adulthood (12-14) the ability to digest it goes away. Just as some people don't have the enzyme in their mouths to start breaking down carbohydrates. We evolved to fill a niche that we may no longer live in.

It's okay, as long as we figure out what works for us.