Just take a bar of bath soap, lather it up or at least try to and shave. You'll see real quick where it is lacking in the properties a good shave soap will have. It would really be a jack of all trades and master at none situation. I can tell you a bar of Ivory soap is not a good substitute for a real shave soap from experience.
"If you want it, that's what you do best" - Woz
"if you ain't bleedin', you ain't learnin'" -me
remember all, each thanks given will ... (virtual ego +1)