While this can be useful in extreme situations, such as a production system that can't be taken offline even momentarily, it has always felt like a footgun to me: a way to make critical changes that are recorded nowhere and flow through no change process. Lisps are full of these really powerful features that should be used sparingly, but are held up as reasons to use it. I wrote a fair amount of CL a couple decades ago, but always felt that "pause, inspect, stop, edit, start" was safer than "pause, inspect, edit, restart". Some corollary of Kernighan's Law, there.