Why would I not use Amazon Cloud Computing
I see no reason not to use Amazon Cloud Computing. The other options are Google, AppNexus, and GoGrid. All seem to be offering pretty much the same services. Here is a in-depth review of all the services side by side: http://www.infoworld.com/d/cloud-computing/cloud-versus-cloud-guided-tour-amazon-google-appnexus-and-gogrid-122
Some reason why you wouldn't want to use Amazon Cloud Computing is it doesn't support Windows 2008, you can't recover an instance and it has long instance time.