Why should PSELx = 0 when the APB transaction is completed?

It would seem that if I let PSELx stay at its existing value at the end of a transaction, rather than unconditionally setting it to 0, it would reduce power consumption, since PSELx wouldn't have to change when the next transaction selects the same PSELx.
This is certainly an advantage.

Nevertheless, why does the APB spec. require PSELx to be set to 0 at the end of a transaction?