Like davextreme, my plan is to put these through HandBrake for playback on my AppleTV 2, and the only way to get "soft" subtitles (you can toggle them on or off) in HandBrake is to use .SRT. I'm not even sure if it can use an external .SUP file at all; it can use the rendered-graphics subtitles from a DVD disc or folder, but only if you want them "hard," or burned-in to the image, they can't be turned off that way.

Plus, I like that I can fiddle with them. On a DVD, or with a .SUP file, if there was something wrong with a subtitle I would just have to deal with it. But with these text-only .SRT files I can actually go in and fix them. I like being able to do that. Since I've discovered them, I'm actually pretty excited about text-only subtitles that I can fiddle with myself. It's not as big a deal in this case as with some of the commercial discs I've seen (sometimes it's like they don't even watch the movie; they just listen once to the soundtrack and write down what they think they hear), but still a nice option to have.