Often it is a bad idea. I get it, though, there is an inherent attractiveness in the idea of starting fresh from a clean slate. Except, more likely than not, you'll soon find yourself in a similar situation to the one you started from. The fundamental problem is that it is easy to underestimate the edge cases. Sure, the main functionality is easily understood and straight-forward to conceptualize. But did you remember to think through all the smaller aspects of your software, too?
Perhaps it's easier with a prototype implementation that in fact doesn't have a lot of features yet, but to completely replicate the functionality of a complex piece of software isn't an easy undertaking. Sure, getting 80% there is probably easy, but the last 20% is the part that's easy to overlook when considering a complete rewrite.
Admittedly, there's nothing sexy about refactoring. And often it may seem like it's less work to just simply scrap everything and start over. However, that's fallacy a lot of times.