There are reasonable circumstances when you'd want these. For example, I might have a daemon where a call modifies some state, makes an async outcall, then modi

There are reasonable circumstances when you'd want these. For example, I might have a daemon where a call modifies some state, makes an async outcall, then modifies more state, and then returns an answer. This function might not be re-entrant. If so, you'll want a lock. This is a fairly common issue in concurrent programming.