It's all about balance of expectation.
In the past, everyone wanted to play Test cricket and the ODIs were a bit of a change. Now that T20 is here and paying good money, there are different ways to go.
England often use the T20 and ODI teams as stepping stones into international cricket.
Hales is down the list to play tests for England, for now. So he goes to the IPL, gets some good experience playing against top opposition and comes back in a better place to play T20 and ODI cricket for England.
He can then build into a test player too.