Mmmmm... well, true, offering oneself to do the time for another's crime is less bad than being forced to, but it still doesn't make sense. I mean - if "fall guy" was a recognized occupation, what do you think the going rate would be to get someone to do a 2-5 sentence in a medium-security prison? etc. (There's a dystopian novel there somewhere!)
Look, I can understand how a believer in God would be comforted by His willingness to forgive us our transgressions if we repent, but many Christians seem to be moved by the symbolism of Jesus the Willing Fallguy. It's that metaphor specifically that makes no sense to me.