There's every reason to root, and only one reason to not root.
Reasons to root:
Kernel modifications to increase battery life. (Sure the Pixel can go 2 days easily on one charge, but why not make it 3?).
Filesystem access. The heart of any computer should at the very least give you filesystem access. It boggles the mind why phones won't give this option when this is a computer that you carry with you every day. Maybe you want to write your own apps, hide personal files somewhere outside of the visible directories like Downloads.
Breaking down barriers carriers put forth to limit you in a walled garden. Whatever it may be. Video codecs for example. Open source media servers that aren't approved on the app stores, I can go on and on.
Only reason to not root:
Android Pay. I absolutely love this ability. I personally think that you SHOULD be able to use this option while rooted. Using SU app, and encryption of your card should be enough to protect your card information. (Don't say it can't because it can.) Rooted apps cannot work unless you give them access via the SU app as added security, and even if they manage to grab your encrypted card information because you allowed some program you've never heard of root access and it steals your encrypted account information, it would take them 1 million years to brute force decrypt it if using a high enough encryption scheme on it. You can just change your virtual account on a whim, and do so ever 6 months, cancelling your old account in case it was ever grabbed making the old one null and void for the hacker if he does eventually get lucky and break it. Your account will never get owned if these practices were followed.
Blatently not allowing you choice to use something like android pay is big brother coddling you like a baby. I'm a grown ass man, and know how to secure my ****. If you don't know how to, then you shouldn't be doing it as your the reason Google puts these sort of restrictions on phones like mine.