Yeah - I dont think you shoudl ever leave a child completely alone as they could slip and knock themselves out.
I think its a great idea to encourage noisy bathtime for toddlers, that way if you need to do something in the next rooom when they are older, you can always hear them, even if you cant see them!!
You may not have to watch them as they reach 3-4 yo, but you still have to check on them IYKWIM?