DO has a very limited API, no ability to add additional storage without resizing your droplets, and has no firewall protection without iptables being enabled. DO has its uses (I'm migrating my personal server there presently), but it's not even close to being in the same market/caliber as EC2.

For some businesses, the huge AWS feature set (RDS, EBS, ELB, security groups, VPC, ELB, EIPs, etc, etc, etc) is more valuable than the bottom $$ line. For others, those features aren't worth the added cost, but hand-wavey "just use DO" or "Just rent physical servers from SoftLayer/Rackspace" is disingenous.

TL;DR AWS has value above and beyond simply hour-to-hour elasticity.