I can present a before and after view of the underwear/no underwear thing.
No underwear 100 times out of 100. It's a better massage on both your leg/thigh/glut area as well as your lower back. It just is.
The only thing my underwear cover is basically the cheeks. Thighs and lower back are both plenty accessible. And don't therapists usually massage the glutes over the sheet anyway?
Underwear (boxer briefs in my case) on, they tuck the sheet in your waistband when they are working on your back. That, and the bottom of your underwear, create basically a barrier to the massage. They won't go lower than that line on your back, and they won't go higher than the bottom of your underwear when you're on your stomach and they are working on your legs.
Underwear off there are no barriers. There's nothing dirty about it, but the long strokes that make a good massage don't have to stop at a predetermined point on your back or legs. Especially your legs. They will work all sorts of muscles almost up through your hip.
It *is* a better massage.