Like many others, I have the dark spot on my 6s Plus, but I certainly never would have noticed it had it not showed up here. It doesn't really matter what I comment on it because either way I'll either be an Apple apologist or a OCD complainer. LOL, oh well here goes...
1) Relatively speaking, this problem is minor compared to other launch issues such as Antennagate. Or even Bendgate since that was something that could potentially ruin your phone if you sat on it in your back pocket. Of course, that doesn't mean it isn't an issue, but I'm personally not really bothered by it (and I'm usually pretty OCD to be honest).
2) The screen on my 6s is otherwise magnificent, with great brightness, contrast, uniformity, color, and viewing angles. I've owned plenty of phones with less than stellar screens, ranging from discoloration to poor viewing angles to brightness issues. So while I would gladly accept an exchange from Apple if the issue is addressed and remedied, I have no plans to proactively seek an exchange.
3) For those who are really bothered by it, you have every right to ask for an exchange or refund, and I'm sure Apple will do whatever it can to make you happy. While they may not address the issue on a wide scale, they certainly will deal with it on an individual basis like they always have. Of course, it may be some time before you can actually get an exchange that doesn't have the issue. I mean, who wants to take the chance with multiple exchanges in order to get a perfect one? But since there's a 1 year warranty on it anyway, even without Applecare, it may be something that can be exchanged in the near future.
So what's this one going to be called? Sigh...
Shadowgate?
Spotgate?