The way that my business works is that we take a large number of orders in a short amount of time and then it tapers off. If my site was down because their api is down for 4 minutes (and I don't know when it is going to come back up), that would really affect my sales.
If you are at a cash register and pull out your credit card and the swiper doesn't work, chances are you can just hand the person cash. The internet doesn't work that way. 4 9's for a payment processor really isn't good enough. Also, that stat is just across 90 days, not 365.
I really like Stripe and what they are trying to do. Now that I can visually see it, I just worry about their uptime performance.
Update: Some one just sent me email calling me an asshole, which seems rather undeserving. This is their ip address pool-173-49-2-60.phlapa.fios.verizon.net.