True, these are the biggest races. There are not many riders who have won a variety of them in the current peloton. Nibali, Gilbert and Valverde are the three riders standing out.
Winning the toughest monument on the calender 4 times ain't too shabby. Gilbert would sacrifice half of his victories for another victory in LBL. For Nibali it is the monument he bases is spring season on. You shouldn't downplay the victories of Valverde there.
I do agree that in thirty years time or so that for a GT/Classics specialist Valverde has just won one GT, and the least prestigious one of them. This means he will not be in the tier of Coppi, Bartali, Merckx etc etc. But his sheer consistency in both GT's and classics in a time many riders only ride either stage races or classics, his accomplishments should never be diminished.
Ask any rider for the most impressive rider of the 21th century and I guess most will say Valverde, Nibali, Lance, Gilbert, Contador, Bettini, perhaps Froome and Cancellara. In this group Valverde might not be the greatest, but it is stupid not to think he belongs in this tier.