The problem is UE has to decode all the RB's in different demodulation. it increase the complexity at UE side and leads to processing
delay. Even at ENB it increase the processing delay, say E.g 100 RB's are using difference MCS the ENB has to code in each RB's so it increase the complexity.
Moreover Each RB's CQI will not vary drastically. it will be sub band CQI variation, so using multiple RB's in the same Sub band will be feasible solution and reduce complexity.
Lets see PHY Expert opinions. :)