But you only unlock SB, ShB and EW once you get the key for EW. Until EW releases, you only get EW-preorder-key, which doesn't have the expansions.
Starter edition removes the free trial restrictions, as good as the free trial is.
I suppose you could say that people should just buy the Complete edition once they want to commit beyond the free trial, but if they're near an expansion launch and want to be out of the free trial, but don't need to do post-Starter edition content yet, it's probably cheaper to buy the Starter edition and then wait for the new expansion to buy the rest.
This is one area where Blizzard excels. They always just have two basic offerings: the the latest expansion, and everything up to it. Also, you can download it to whatever supported device you're playing from. There's none of this nonsense with having to buy a copy per platform or having to purchase previous expansions.