As a small business owner that lives in a small town in the middle of nowhere, that $80 a year makes perfect sense.
Being able to order computer parts with free two day shipping is a huge boon. I can't buy them locally (as in, they don't exist in a 130 mile radius), less downtime makes my customers happy, and free shipping increases my margins.
I place anywhere from two to ten orders a week with Amazon. First name basis with my UPS and FedEx drivers AND their backups. Amazon Prime has been tremendously helpful to getting my business going.
Sure, if you live in a metro area surrounded by Best Buys and WalMarts it makes less sense, but many of us don't.