Two of my clients have found that this feature makes the AP module fail to work as part of their accounting process. One case is outlined in this thread.

In the other, they need to change the department only when coding expenses (the account-dept pairs exist in Ledger).

As valuable as the validation is, clearly this is a case where "one size does not fit all" and it should be made optional. For the users adversely affected, that should happen with a service pack as quickly as possible.