A bit of downtime is certainly not a catastrophe, but it might turn into one if you lose a bunch of data because you weren't properly shipping it elsewhere.
A much more sane response to the overwrought fears of EC2 downtime. If you don't know your hourly/daily income rate (often $0), then you can't even correctly value uptime.
Thats just it. Downtime is expensive if you rely on your web site to make you money. If you are a small eCommerce shop with all your eggs in the EC2 basket, downtime can be expensive.