DR's disadvantage is that ammo packs give you something like half as much cloak time than with IW, and decloaking is super loud. I think the damage reduction compensates for the spy's status as the overall weakest class (low health, low firepower, medium speed). It's not hard to kill a (mediocre) DR spy if you know how to play DR spy, especially as pyro, even with the damage reduction. It's basically a joke how easy it is to kill spies as pyro. Anyway, the advantage of DR is that you can use the other things in this poll (sticky spam, minis, snipers, etc) to get behind enemy lines, which can be very useful particularly in pubs and spammy maps like goldrush. It also makes the spy a much more dynamic class than the usual routine of: cloak, disguise, sneak behind/above enemy, decloak, stab, because with the DR you have to rely on acting skills a lot more. I do admit the DR + spycicle combo is quite powerful, though, if you stay away from the pyro. People have complained to me about it multiple times in lobbys/pugs.