I always secretly pray for another Watch book, so right now I feel as if my good luck has to run out some time - we've been getting so many.

I think they (along with the stand-alone semi-Watch books) just provide the most scope for storytelling. Pterry is on record as saying he can't really think of anywhere else to take Granny Weatherwax after Carpe Jugulum, Rincewind has found a niche in the SoD series, and Susan needs a threat to the existence of the universe to set her books in motion. (The trouble is, Death/Susan's traditional enemies are the Auditors, and they've been...done to death, a little. They need a new threat.)

But Pterry could write a history of economics in nineteenth century Romania and we'd still read it. (He could probably make it interesting, too.)