As many manual, the longer PR, the better but in this case the philosophy, in each field, was to make it light and accurate wherever the shocks it may take.
Thus, it leaded RM to make it simple, with a fewer number of parts. Bigger power reserve would have implied bigger spring barrels, hence, more weight. It wouldn't have been many grams but I like when they do it at 100% not 95 personally.
Of course another issue is that it would have also meant to design a new system and I guess they didn't want to in order that the price doesn't inflate even more.
Thanks for your input gasterus,
Cheers,
Mark