Why not root?
Because it voids the warranty.
Horribly overrated statement.
First, there is no Knox or any kind of flash counter on a Nexus... You can revert any phone back to stock as long as it turns on... even if the screen is junk and doesn't light up (if you can get it blind-loaded into the bootloader). But if the phone is utterly trashed, no one is going to bother worrying about if you rooted it or not, they are most likely going to bin it anyhow.
Second, it is damn near impossible to perma-brick a Nexus to the point where you can't, at minimum, flash the factory images via 'fastboot'. If you can get it into the bootloader menu, you can fix it... and even then.... there's a backup bootloader on the odd chance that you screwed up the primary bootloader (trust me on that one... long story).
So you can get that modified Nexus back to bone, 100% stock... locked, stock kernel, system software... you name it... Now, I am sure some forensic digital media expert could then examine your phone and determine that you may have modified the device, but neither your carrier, nor Google and Motorola, cares enough about you or what you've done to go to that step.... that kind of stuff is reserved for the FBI, etc.
So if you have to return your phone because you cracked the screen, or the speakers stopped working, or the radio fried... I guess the only scenario where you'd be SOL would be if you broke the USB port and your computer can't connect... then I guess I'd take out my trusty hammer and put it down like Ol' Yeller.... but I actually haven't heard of something like that happening recently.
So yes... if you waltz into T-Mobile and hand them an unlocked, rooted Nexus 6 running a custom ROM, yes.. they might void your warranty, but then that would be your own damn fault for being stupid.