It's all about variety more than anything. When I first started brewing, I'd make a batch and not start another one until it was almost gone. I realized that I'll be in the mood for certain beers. Some days I want an IPA, some days a stout, some days both. I like having that option when I go out to the keezer. Once I get a couple more batches done, I'll have 4 taps and probably another 6 or 8 commercial varieties available at any given time.
I am starting to consider cutting down on buying commercial beers though. I want to try everything, and for the longest time I was buying different beers almost weekly. I though it was necessary to try every IPA I could get my hands on. I'm starting to realize that while variety is good, instead of spending $10 on a 6 pack of IPA, I can spend $20 and make a decent IPA. This really hit home when I spent $14 for a 6 pack of Bigfoot Barleywine recently. While having a big beer cellar is a future goal, why can't I just make a good beer myself, bottle it, age it and enjoy it like anything else. Spending big bucks for beer doesn't bother me, but if I put that money towards ingredients I could probably get away with brewing more often, and having just as much variety. I can save buying the commercial stuff for when I go to a pub or on a special occasion.
OK, that was long winded.