I love putting recurring opponents in my games. Problems come up when players don't believe in the concept, or when the PCs turn out to be just too damn effective. I think the worst time was when a source turned out to have had some extra info the PCs didn't get on first pass, and they went back and tortured the source to death. Short-lived game.
The phrase "overzealous" comes to mind, there. Or possibly "sloppy and kinda psychotic".
Personally, I have all my NPCs tagged "disposable until proven otherwise" in my mind. Like, for example, there were two Orks and a Dwarf (Big Tony, Larry, and Shep... guess which one was the dwarf.

) who were the guys in charge of the crew of Halloweeners standing between my players and their goal. Two survived. Now I'm going to keep bringing them back until they're killed off or... and I highly,
highly doubt this one... they
* somehow manage to TPK a group of PCs that are on S-K Prime's "full-time employee" payroll.
I'd go into details, but at least
one member of my table is on the board, so I don't want to spoil it for him.
*I'm a firm believer of the "I just narrate the thing" school of GMing. Once I have an NPC created they're gonna do what they're gonna do, and I just tell the players what's happening... with the occasional thumb on the scales to make sure "plot" happens, and things stay fun, of course.