The supposed main benefit of tubulars is ride quality...e.g. a totally round tubular profile gives a more supple & true ride than a half-round clincher. Also, they typically leak slower when punctured - important I guess for pros descending alpine passes. I think #3 would be that they don't pinch flat, but I've never actually seen a road clincher pinch flat anyway. Of course, I don't ride cobbles at 30 mph where that may actually happen.
The main drawback to tubular would be the pain in the butt factor...stretching, gluing, having to carry a whole tire for a spare vice just a tube. Oh, and they are known to roll off the rim while cornering, putting bike and rider firmly on the ground. Did you see Beloki in last year's TdF? He's still not right and it's been over a year now...I'll bet he never descends with complete confidence again. I can imagine myself mounting a new tubular and then spending the next 100 miles wondering if I glued and cured it properly....yeah, that's what I want to think about when I ride.
Weight is almost a wash and a small enough difference not to matter. I'll bet you can find thread after thread about tubs v clinchers on the roadie side if you really want to pursue it.
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Rock Lobster