I think the primary disadvantage of a quilt is that if you roll around a lot they tend to roll off you. I haven't used a quilt, but I have used a sleeping bag with no insullation on the bottom. It's a little bit finicky, but it works okay. It lets me take a smaller pack which is really nice when climbing/mountaineering. For something like the WCT, it's certainly not necessary. But if you're a very calm sleeper, or you don't mind fiddling around a bunch in bed, they can be nice to have.
Regarding temps: 30 F is probably fine for the WCT in July, you can always put on an extra jacket. Be aware though, that some nights are much colder than others, so don't put too much weight on one experience. It's easier to let cool air into a warm sleeping bag than to make a cold one warmer.