I was in theater last year talking to the guys who ride in the back of the CV-22. One of them told me why they can't fast rope, it had something to do with the exhaust when the pods are vertical or the fact that they CV-22 has to hover too high to stay out of the vortex ring state (I can't remember). I do not that after the accidents a few years ago the CV-22 had to maintain forward translation when close to the ground because it might have been more susceptible to the vortex ring thing. These are guys who would be fast-roping into very small/tight footprints, so precise positioning lower to the ground would be nice.
Several of them relayed how they couldn't see the aircraft in the LZ due to the dust storm caused by the exhaust pointed at the ground - even with the blade pitch flat. One busted his leg when he ran into the ramp. Also, they had to leave certain equipment behind due to the smaller compartment. Bottom-line, if the guys in the back can't take their gear or infil/exfil like they need to, it does not matter if they get there faster.
From the operator point of view, not many RW guys that I talked to (who were likely to transition to the CV-22) liked the fact that the CV-22 can't glide nor auto-rotate in the event of a dual-engine failure or transmission failure. One engine can drive both rotors in the event of an engine failure if the power transfer system works, but if there are associated transmission failures all bets are off.