The stupid, it burns.
My dad-who is 63-drinks an Ensure with his breakfast every morning. He has a very physical blue-collar job, and the vitamins and such in that drink help to keep him (relatively) healthy and keep his energy up. When he does get sick, he pretty much exists on Ensure and chicken noodle soup because calories are necessary to fight off illness. If the day ever comes when that's all he can handle, I'm 100% sure he'd find Ensure more palatable than any sort of flaxseed-wheatgerm-the air in my mouth tastes better than this crap-smoothie.
My kid also drinks the Boost for Kids and Pediasure, as recommended by her pediatrician. She's a skinny little thing (not even 25lbs at 2.5) and a picky eater to boot; the doc recommended them not only for caloric intake, but also to help me make sure she gets the vitamins and minerals she probably isn't getting because she subsists solely on grilled cheese, PB&J, and milk.
The OP DUmmie misses the point...they are not replacements for meals, they're supplements. And you can bet your ass that when I am old and missing my teeth, I'd take an Ensure over whatever nasty-ass holistic-organic-natural crap they'd try to feed me.