FWIW, I used to go down to Lake Powell with a large group of friends. Most of the group had Springbar, but some had nylon type tents and some used the nylon tents for their kids and they'd sleep in the spring bar tents. Well, there would be some freak wind storms that would come through and would shred any tent left up besides the springbars. This included a couple nice North Face tents, a couple MSR tents, etc. It only took a storm or two for everyone to learn not to leave their tents up when one of these storms start to come through. I don't know what it is with the Springbars, but you could leave them up and not have a single problem with them. When these storms would come through you would run to the houseboats or to a springbar for shelter.
I've also been in torrential down pours in Springbars. The most you would ever feel is a slight mist in the tent, not even enough to get anything wet, and it may have just been the cold air but you could kind of feel it on your cheeks. I'm a firm believer in the springbars if space and weight aren't major concerns and you're able to use stakes. If you get one, first thing to do is either get extra stakes or buy some high quality metal stakes. They come with either the crappy yellow plastic stakes or the metal with a plastic tab on the top (can't remember, I do know that's the first thing you should get though due to the necessity of the stakes with them).
