Because it's not a common unit for comparison. The spot price can fluctuate. Obviously.
(you edited your comment, so I'll edit mine :) ...
How can you assume the spot price will stay the same?
(you edited your comment, so I'll edit mine :) ...
How can you assume the spot price will stay the same?
How can we assume Amazon won't raise base prices, possibly affecting the spot market, then again this is a spot market! It's like asking how can you assume stock prices (not exactly equiv, but thats not the point) will go up or down?
How can we assume anything?
We don't. I think the assumption on the wrong feature to market is the problem here.
I only care that right now, EC2 spot are cheaper by a large margin. IF the price raises to match then I'll adjust. I'm not locked in.
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.