A fair comparison for what exactly? Many EC2 users optimize price for their usage characteristics heavily with spot instances. Sure, a c1.xlarge costs me $0.64 per hour, but I probably only need a few of those (at potentially a reserved price point) as a baseline. Instead of scaling those up and down, we choose to scale up and down with the c1.xlarge spot instances at a magical $0.052 per hour price point and only switch to normal on-demand instances when spot aren't available or the prices spike out of our target range (which, historically, is rare).
Bottom line, comparing $0.64 vs $0.64 is fine for per-unit usage but doesn't paint the full picture when it comes to actually building scalable systems on EC2. Spot instances play a large role in that strategy.