old shriveled backbulbs without roots don't need to be left on since they will not really contribute much to the plant, assuming that there is a sufficient number of viable leafed growths. I take them off because otherwise you start to use pots that are too large for the root mass, and because the old ones are pretty much useless to the plant. I don't see them killing the plant though. An infection can set in an old bulb and then progress, but then again an infection can set in anywhere.