what does jekyll see as the value of seperate identities?

This is similar to your question before. I think Jekyll always felt he could party as Hyde for a while yet retain the choice to go back to his mild mannered identity. Everything, however, exacts a price. Like a person who dabbles in highly addictive drugs, the threat of bondage to his dark side is always near. This is a theme throughout literature and movies. Hey even Luke Skywalker was almost consumed by the dark side!