How far is your commute? For the last two years I've been commuting 5 miles each way a few times a week and I'm starting to show improved strength. I'm normally a long distance runner and occasional fitness and longish distance rider (45-60) miles, but I am a spinner and pretty much always sat to climb, in part because I would bonk if I climbed while standing. Now, I pretty much ride my commute as fast as I can (14-16mph avg in traffic depending on route and time of day), but I feel much more comfortable getting out of the saddle to climb and my 16-20mi weekend route is showing improved times overall. Anyway, just one person's anecdote and certainly not a scientific one, but I'd say if you ride hard, at least some of the time on your commute, it will increase your fitness.