As others have said above, if you purchase a more expensive flashgun with HSS, the flash sync issue becomes a non-issue on the 6D (as long as your flash is powerful).
Where the 1/180s becomes a more limiting problem, will be for those owners who purchase a cheaper flash that does not support HSS. In those instances, especially shooting outdoors during daytime where the ambient light is strong the photographer may need to use as fast a shutter speed as his equipment will allow, therefore 1/300s or 1/250s may be better than 1/180s depending on how strong the sunlight is.
[Remember your aperture setting controls the amount of reflected light or flash, whereas the shutter speed determines the amount of ambient/everywhere light in the exposure - thus adjusting the ratio of flash to natural is done via playing Aperture vs Shutter]
Notwithstanding all of that, if a prospective 6D owner can afford to pony-up 2 grand+ for a camera body, then they can certainly afford to pay another 500 bucks for a decent flash unit like the 600EX
Also remember what Neuro said about shutter mechanism quality (after all we're talking moving parts in an expensive electronic device here) - the 1DX is rated for about 400,000 shutter cycles, so clearly has the best quality parts and a decent x-sync rate, whereas both the 5D3 and 7D are rated at approximately 150,000 actuations (with the 7D mechanism being a modified version of the 1DIIn & 1DIII) hence the 1/200s and 1/250s x-sync speeds respectively. Ergo, given that the new 6D is ONLY rated for 100,000 shutter cycles - the same as most Rebels, Canon obviously believe that the 6D shutter is a bit more fragile, so do not want to stress it too much as well.