Well, for one thing, it would be pretty expensive, and I imagine we would get pretty tired of them after a bit. They are heavy in calories and salt, etc. Consuming MREs for a long time would be about like eating at McDonalds regularly.

The newer varieties are pretty good, especially given the constraints under which they are produced and used. Nontheless, I can easily produce meals that are lighter in weight, nutritious, and cheaper, while being relatively easy to prepare - by that I mean that I need to boil water, about the limit of my culinary talents. I would never take MREs backpacking.