Mark, I would just like to report our story, as repects to usage being a cause. Our 7900 clogged on the yellow channel after 2 years and 1 month. Our use is low, it shows 1350 prints, we work 4 day weeks, so it's off every weekend from Thursday night until Monday morning, it's turned off for vacations once in the summer for 3 weeks, once in the fall for a week and for 1-2 weeks at Christmas. I know there are always exceptions but I'd say if light use or being shut off caused clogs, ours probably should have clogged long before now.
No amount of cleaning has improved it, if we had a 1 yr extended warranty we would have exceeded that by one month, if we bought a 2-yr that would have been half the price of the printer to start with and after being repaired you still have spent $2100 for a head replacement and have no warranty going forward. Best to us to just replace it now, get at least a 1-yr extended warranty, be covered for next 2 years, keep some cleaner on the pads, don't let ink carts stay in for more than 6 months, print a nozzle check every day so at least a little ink moves, and hope for the best.