They are worth about 5 PDs last I've seen.
I'd keep them if it's your only one though. The S/D the blades give can make a difference in some situations if you care about speed at all, or don't have ready access to someone to cast higher levels of it. They are among the strongest daggers, though there are certainly stronger or more effective weapons, so it is mostly used for its special before switching to a more appropriate weapon.