Well if it works, it works.....
But I do find it hard to believe it will be reasonably breathable and durable. I also think of it as an all or nothing proposition -if it leaks somewhere, you're probably going to suffer mightily in the rain, whereas at least in a leaky tent you may be able to salvage a reasonable setup to sleep. Other things:
i. no privacy for getting changed in the morning (course, that depends on where you camp, but if you do camp at campgrounds it's probably going to be a problem sooner or later).
ii. it's raining and you want to read a book/write a journal/rearrange pannier contents, etc.... how are you going to do that?
iii. ever heard of Chinese water torture?
ivi. you'd still need a covering to prevent insects getting to you -might be fine, but I wonder if sleeping with effectively a mesh bag around my head would be too annoying for me!
v. and the big one: you might be mistaken for a big maggot.
Huuuuuge caveat here is that I've never used one, so take these comments for what they are worth. By the way, if you are that concerned about UV, why not take a tent and then set a tarp over it? I'd bet that would give it quite a bit of protection. And if you are going to be at the same place for quite a time, why not just buy a cheap large tent that does offer UV protection (with as much ventilation as possible) and leave it up? Even if it suffers damage or is stolen, at least it didn't cost you much. Also, if you are concerned about UV protection, wouldn't that be largely negated since you (or at least I do) only go in a tent mostly at night to sleep? Maybe I'm uninformed, but at least for bike touring, UV protection for my tent is low to non-existent on my list of priorities.