I know Honda used the engine as a stressed member to support the rear swing arms on the 954 but it isn't on the new CBR1000RR's. I think it's day has gone past. I believe they were saving weight by having less frame and beefing up the engine cases but with new metal technologies and other light weight bits, they can get as much if not more with a full backbone frame. I do remember when full perimeter frames went away, looking at the backbone frames and thinking..............."no way am I racing that"!
Now it is not just about being lighter but where you are carrying the weight. "Mass Centralization" does so much more than taking weight off, making the bikes flick from side to side effortlessly, not only making them turn quicker but with less effort and fatigue to the rider.