The question to ask and answer is - "What is your final output?" If your taking pictures that will be printed to 8x10 or 11x14 or "normal" photo sizes then do you need the absolutely best IQ lens there is? Becasue you will not see the difference between the Nikon and Tamron G2 at those sizes. Even less so if your using the photos for online viewing! If you have a camera sensor large enough to be making poster prints then the lenses might start to show a difference. Maybe! Especially with lens and camera corrections in software now. Many lens faults are now corrected in post. So, there is even less that can be discerned when comparing "normal" sized prints.
If your output will never show the small quality difference between lenses. Why pay the extra? Now having said that, the majority of my AF lenses are Nikons though. With about 1/2 purchased used. And that's not for absolute quality though. But for compatibility reasons. Although I just recently ran into an issue with Nikon teleconverters that are NOT backwards compatable! And new P lenses are not backwards compatable. So, even Nikon is moving away from full compatibility (at least backwards that is)!
That's a good point. I'm sure the G2 will be a fine lens. But I know myself, and I know I'll feel some type of way knowing I could have had the best lens, but don't. However, the TAP console seems to assure people that compatibility shouldn't be an issue, but durability is yet to be seen, since it's so new. But I got my hands on it yesterday in-store and it feels pretty solid.