Can't see how this would be more important than the poor midi sound.
The mere fact that you have described the sound engine of a modern sound engine as the "midi sound" shows that you are living in the deep dark past with respect to these instruments. You need to update your knowledge of them to be commenting so authoritatively.
Being 72, I always live in the past. My first in 1987 was an 88 key Kurzweil but more recently I have a Yamaha Tyros 5, 76 key and use Pianoteq 6 Standard version with Steinway B and BlÃ¼thner Model One add-ons.
All of these were, and are good sounding instruments but are no match for my BlÃ¼thner acoustic instrument. I may be guilty in saying "midi sound", but if I had said "sampled or modeled sound" it would still mean that the sound is inferior to the native acoustic sound of the hybrid. So my opinion about the let-off being of lesser importance in achieving the best sound stays relevant at least to me.
I'd say it all adds up and every bit counts. Of course, I prefer a real acoustic grand myself over a hybrid, but I think we have to recognise how far the sound engines have come in these hybrids - Yamaha has 4 channel sampling now, more sophisticated resonance modelling and they even made a binaural sample set specifically for when using headphones. Whilst it might not be a grand piano yet, it is getting remarkably good and very musical. Some fine musicians are using them now and stating that they are very good pianos to practice with and they have minimal adjustments when switching over to a real grand.
With that in mind, I think even though the sound is not quite perfect yet, the predictability of the action and the comparison to the response of a real grand becomes a very salient matter, so parameters like let off become very relevant because it can determine just how authentic the action feels and how the sound engine is triggered - which has implications for pianists who use them as practice workhorses in preparation for recitals on acoustic grands. So, I do think that it is a relevant question regardless of whether the sound is perfect at this point in time.