I can't speak for others' reasons for using smoothies, but I feel like I have a pretty good one. There are areas along my digestive tract that for whatever reason are susceptible to blockages -- scar tissue, strictures, whatever. My best guess is that it occurs at the pouch's inlet and is referred to as afferent limb syndrome.
I've also had chronic pouchitis that's becoming antibiotic-resistant and am starting Entyvio soon (please God let it work). My issues could all be pouchitis-related, but I'm fairly convinced much of the pouchitis occurs because of the partial obstructions I get. For that reason, I stay on a soft foods diet and prefer getting veggies via smoothies.
It's no fun, but the times I've had fruits with seeds, salads, and other things, my stomach will bloat and my lower-left abdomen will be in varying levels of pain. Over the last year and a half, I've gone to the ER twice when the pain became unbearable.
The first time images showed areas where food was backed up and my intestines were stretched so they pumped my stomach with an NG tube. The second time, I was able to throw everything up before getting the tube put up my nose and down my throat. I threw away my vomit-covered shoes, walked to my car in my socks, drove home, and stayed on a clear liquids diet for a few days. Since going on a purely soft foods diet, I continue to have pouchitis issues, but I haven't had any pain -- knock on wood.