I commute during summer though not consistently as I'd like; its really helps with the volume but I tend to leave the actual training til I get home. A lot of others use it for training; for example Charon Smith only trains during his commute and he's one of the best crit racers in the US. Volume is important as well and 8 mile trips should probably be added on at least 1-2 times a week if that's where you are getting all your cycling from.

A huge benefit to commuted is its easy to ride fasted. I take my breakfast with me and eat after a 40 minute ride. Weight drops fast that way.