To be honest, yes, I think it is legitimate to a certain extent.
It's always difficult to evaluate Civ in terms of hours of gameplay because the value is in replaying using different Civs, different strategies and different victories. Both of the expansion packs give a massive increase in the number of replays you can do, by introducing new civs, and by bringing in big changes in the strategies. I'd say that I got more hours out of G&K than I did the vanilla game, and the same again out of BNW.
The counter-argument to that is that one of the reasons why G&K gave so many hours of play is that the vanilla version had flaws, and one of the reasons for getting more hours out of BNW is that the Diplomacy victory STILL sucked, even after G&K.
Regarding the new Civ, it's a different game (other-planet colonisation, and a cross between Civ and Alpha Centauri). I'd say the only reason to hold off because of it is if you prefer that to Civ itself, or because of the possibility of price discounts.
@Glaroug - Morocco trading was my first playthrough on BNW, the first time I'd ever enjoyed a Diplomacy strategy in Civ V. I prefer Arabia as an ICS Domination victory though, the camels are awesome. And my all-time favourite is still the Aztec Cultural Genocide Victory, which was only possible in G&K.
You guys are really tempting me to give up on Burial at Sea and play Civ again.
