Sacrificial Pit – how to use it correctly?

Of course, it’s better to do it this way:

don’t send him to any pit, but leave him in the garrison, or, even better, give him to a new hero

On small maps, Warriors are essential, and on large maps, you really need more heroes capable of finding their way to new neutral castles, artifacts, and mines.
Unless all the heroes available for purchase suddenly have a large number of troops, preventing them from taking over neutral creatures that happen to be in the castle... well, then I think you can send them to the sacrificial pit.