I like the idea of having fewer doohickeys and whizbangs on the DSMB. The main complaint I have heard about the open-bottom bags is that the tend to leak air or take on once they reach the surface, and once they do, they fall down. I've never tried an open-bottom one though. Have you found this to be the case with yours?
I haven't really had that problem. I mean, they don't go shooting out of the water when you launch it from depth. You do have to keep some tension on the line to keep the bottom submerged, but a few seconds as you're fiddling with something else isn't going to do it. There is still the weight of the clip to keep the bottom facing down.
Once you are at the surface, if you need to keep it filled, it is a bit more of a pain than a closed one, but still not horrible. I'll trade that for ease of stowing.
Where is isn't good is if you need it for surface floatation. You aren't going to inflate it and give it to a struggling diver or swimmer, wrong tool for that job.