I don't think chopping and changing is a good idea, but speaking purely from a Test perspective the batsmen need to know they do not have a divine right to play. The Bell being dropped from one day is partially down to his recent one day record, but also I imagine to give him a mental break. He looked completely shot at the end of the Test series and a bit of time to refocus may benefit him.
well we do write players off easily don't we. Bell scored 950 test runs last year