Oh well sure it can't on iOS devices, but that is Apple's doing, not Google's.
It does have full functionality on all GMS (Google Mobile Services) Android devices though (Sammy, HTC, LG, 1+, Sony, etc.). As for people not buying a device for it's assistant, I think that is an extremely short sighted comment. At least in the US where everyone has an iPhone and considers Siri near worthless, not many people use their assistant. And many of those that do are locked into a mindset that it can only do basic things/answer simple questions (i.e. remind me of x, whats the weather, whats score of the game, text nick hey). My friends head nearly fell off the other day when I asked my phone why people put nitrogen in their tires and it read back a perfect response without me even having to touch it.
As people break out of the "assistants are shitty" mindset in coming years, and especially as talking to your phone becomes more socially acceptable, I think there will be much more focus on who does it best. Without a doubt the future is going to be riddled with AI. In that sense to write off personal assistants as marketing gimmicks sounds as ludicrous as writing off the electric light bulb as a neat trick in 1850.