How often, how loud, and how long you play your systems will greatly influence the alternators ability to keep the battery charged.

I think that I experienced my alternator problems because I was traveling accross state several times a week when looking for a house. I was cranking my system the entire time. I put 10,000 miles on my car in 2 months. I think the alternator needed to work real hard at all times, and I think it was this that degraded my alternator output. Most people don't do this extreme amount of alternator abuse and therefore will probably not experience problems.