Mike Spence wrote (see)

Lighter wheel?

Trying to change clean tubs is a mare, it would totally rule out mid ride/race changes, but I guess in the current days of pits and spare wheels there's more advantage.

Quite a few riders have been doing it for a while, lots of Trek riders were using Bontrager Race X Lite Carbon 700c rims using custom Dugast tubs with Hutchinson treads on them a few years back.

i picked up a pair of tubular mtb tyres (not bought a pair!) at the grand raid a month back. jeez they were light... think they were ~350g all in (compared to a 350g tyre, plus tube), and i bet the rims are lighter too...

for pros with decent pit facilities, on a course which isn't prone to puncturing, i can see why they might use them.