I think using a snorkel for part of your practice is a good idea. It lets you focus on other elements of your stroke. But I would never suggest using it for a whole practice. Maybe start out with a few 100 yards of easy swimming, then do a few 100 yards of swimming and drills with the snorkel. Then do the rest of your practice without it.
I personally HATE the snorkel, but I use it once in awhile because it helps me get a better look at my catch. Some of my friends have made big improvements to their stroke while using it. They're also really popular with age group teams.
Just don't be one of those people who swims for 45 minutes with a snorkel.