On a standard 200mm DH frame, a dual crown 200mm fork is the norm, however i have seen a few folk with 170-180mm single crowns on the likes of their spesh demo or trek session, Apart from the fact you can x-up with a single crown, what are the advantages; Weight? More Manouverability? Obviously a dual crown would be more rigid. Why the single?
More maneouverable, mainly - not much steering lock on a dual crown for tight switchbacks. A2C measuremernt is generally the same on a 180 and 200 fork, and weight savings aren't huge. Guides I rode with in Finale Ligure this year rode that setup - one had Totems on a Demo, one had a TR250
Singles feel far more nimble generally and move around easier but I run duals as I prefer the stiffness off drops and I think I can push harder through rough stuff.
single crowns are much easier to put in the back of your car with the front wheel out!