I get the idea... it's convenient, it's "complete", etc.
And there are obvious downsides (dental being the one that springs to mind most, along with insufficient variety for your gut bacteria, etc.).
But what I don't understand is when it's pushed as a "cheap" meal replacement. Like a sandwich is some horrendously expensive item if you make it yourself.
I mean... to be honest, they go about the marketing all wrong. Sell it as a survival food to camping stores, then as "what astronauts eat", then go for the health lines. Sell it at a premium, too, rather than claim to be cheap but actually be quite average in price. They also sell on "it's quick to make"... which I just find odd.
I'd be perfectly happy to live off space-meal-pills, to be honest. It would be way down on the list of my concerns, even if I would miss the occasional chip-butty. But... I don't know. It's got more in common with a body-building protein powder to me than "this will save you having to actually bother to cook".