1. 2 more cores are pointless if they are not used. Modern games do not show any benefit going from 4 to 6 Intel cores, none at all.
2. No it doesn't. There are plenty of CPU bound games - WoW (and other MMO's), Arma3 to name just two.
3. A 5820k can't run two GPU's in x16 x16 mode, only a 5930k or 5960x can, so Skylake is identical to a 5920k for 2 GPU uses in terms of PCI-E lanes. Skylake can also support a 4X PCI-E SSD from the Z170 chipset, thanks to DMI 3.0.
4. Do you really believe game developers will dedicate resources to make games perform better on 6+core intel CPU's, when less than 1% of the PC gamer userbase has a 6+ core CPU? The answer is no - they develop for the mainstream, not for the enthusiast.
5. This is only a benefit if the user is planning on 4 way crossfire/SLI, in which case he needs a 5930k or a 5960x to actually run 4 GPU's well, as a 5820k doesn't have enough lanes to do so (neither does Skylake).