wickedfan, I think the difference is in what is expected from the material.
Audiences expect "Soliloquy" to be sung perfectly. The song is part of the musical theater canon and in some ways sacred. Hayden could do whatever he wanted with Billy's other music, but "Soliloquy" had to be perfect. It's what audiences expect.
It was like Tyne Daly in "Gypsy". Some people were disappointed because she wasn't up to singing the score. People come to hear "Everything's Coming Up Roses" sung perfectly, not going flat as Daly did several times.
There are certain pieces of theater music that need to be sung properly or not at all.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.