Back when I was writing a lot of Common Lisp (CMUCL), the image system didn't seem like a win over files, but a misfeature necessitated by the bug of taking 20 seconds to start a program.  Images are inherently messy; after you've changed a running system two or three times, it's even odds whether it will actually be the same as a system started from scratch with ostensibly the same variables and functions, in my experience, and there's no good way to be certain, besides restarting.  Since you're going to have files of source code anyway, using runtime images just creates the opportunity for your two official copies to get out of sync.  This isn't worth the few times you could hotfix something instead of restarting the system.