Is it profitable to use Amazon EC2 to mine litecoins?
Yes it can be worth it, but generally what happens is if it ever is worth it then a huge number of people start doing it until the prices of EC2 are driven up and it is no longer profitable. Here is a good blog post about the subject:

http://da-data.blogspot.com/2013/12/briefly-profitable-alt-coin-mining-on.html