The main reason it's grounded is I believe from what I have read is because the engines are at their life limits, they I believe had already been extended once, but with no viable replacement units available RR took the correct decision and withdrew the design authority for them, that would have resulted in the others DA's being withdrawn.
It's all well and good saying let it fly etc, but if it went down at an airshow etc I would hate to think of the casualties and in this day and age the company share price is everything. Just look how quick RR were to release a press statement when the BA 777 burst into flames in Vegas pointing out it was not their engines.