Personally I really like flavor in mafia games. Kind of obvious, I suppose, but I think it makes it more interesting to play. Would we have done 18(19) Paranormals by now if it was just the roles and the flavor was simply "You shoot webadict. He dies."? Probably not.
I also enjoy writing flavor, so I suppose that's part of why I've run so many.
That said, you have to be careful with Flavor. Ideally you want it to make sense and get people into the story. But you also have to be careful that the flavor doesn't give away too much information. Generally people don't know who did what, and poorly thought out flavor can give things away. Worse, it can give people the exact wrong idea of what happened and throw off people's games. And if you're inconsistent about how much information gets provided you can effect the outcome of the game unintentionally. I'm pretty sure I've done all three of those at some point or another.
A good guideline for writing flavor is to give yourself rules on what exactly is known for every action. A cop, for instance, should only know the Alignment of the person he inspects, not the Role. So don't give away hints as to the person's role when writing the inspection flavor. A doc shouldn't (generally) know who attacked the person they save. And the person who is saved generally doesn't know who protected them. Make sure you write your flavor with the rules in mind, and don't break the rules. If you really want to break a rule, make a Role that specifically breaks it. For example, in Paranormal I have a guard role that dies when protecting but tells the person they protect who attacked them. Since I know what the role does, I can decide how to weight it in terms of power and that lets me balance the game around what can happen.