Yes, one can live on MRE's for extended periods. I have been impresses at the variety and general tastiness of current offerings, which are far better than the C rats they replaced (and even those were pretty tasty when your options were limited). But MRE's are rather heavy, pretty close to canned goods in that respect.

It seems like the point of the project is to provide good food (defined as nutritionally adequate and at least reasonably tasty). Weight will be a factor. And just like the NOLS experience, they are finding that providing basic staples and enabling variety will provide good nutrition at less weight than preplanned and prepared meals. MREs are great for their purpose, but extended forays into space is something different - one of the differences highlighted in the piece I heard was the universal desire for more spicy food.