I would think the point of the monster-slaying sidequests would be the fact that that is what Witchers are for in this game world, and how they make their living. (please correct me if wrong )
It makes more sense for the Witcher to be slaying critters for cash than carrying the letter of the beautiful damsel to her soldier on the front lines or whatever other fed-ex format sidequests are usually produced to get levels and loot.

Edit: Very nice cinematic, Wulf. Beautiful artwork.