I thought this but after doing some tests the results are pretty negligible.
A windows 2008 server license on 2 machines for fall back comes in at around £700 - £800 a month per instance. The same redundancy on Azure is around £80 - £100 a month. If it doesn't work then we go back to the drawing board, if it does work then we have saved ourselves an almighty chunk of money.
We cache a lot of data in our application too which we are putting into elasticache, which is local to EC2.
We haven't used it in great stress yet but initial tests were very good