You can save a room to your library with an elevator and then place that room any amount of times needed.
The issue with placing elevators on disallowed lots (or more than one elevator anywhere) is that you cannot return to Live Mode from Build Mode without removing them and you also cannot save in Build Mode. So any lot editing would need to be done via Manage Worlds or you would have to exit without saving from Build Mode.
They work fine as long as you remember that restriction though. Also, sims will spawn in front of ground floor (or possibly basement) elevator instead of in front of the lot, as it becomes the default portal.