You may be right. However, I also read a couple articles (I'm not going to try and find them) that quick charging may be bad for the overall longevity of a battery. For those of us that will be keeping the Nexus 6 for a long time, it may not be the best choice. I could be wrong, but I'm not willing to take that chance.If used occasionally, it is unlikely to have much impact on battery life.
I think a lot of hard core users here will probably get a new phone before battery longevity becomes an issue. Whether or not quick charging is a potential problem or not is a moot point to me. I always err on the side of caution, that's all.