Rounding rectangles is superfluous fluff, but it also is nice, and serves a purpose in the context of the whole design language they're using. I'm not against rounding rectangles and other such UI fluff in general. But I am against throwing away perfectly working controls, with all the ergonomy their offer, and replacing it with a half-broken, slower version of that control that only works if you use it in one particular way, but hey, it has rounded rectangles now.
And I wouldn't write off rounded rectangles as superfluous fluff. They're fairly ubiquitous in user interface design because round cornered structures are fairly common in nature. They make a UI look more "real". And decreasing the artificiality of a user interface isn't superfluous; it lets more users interoperate with the interface without feeling like they've strapped an alien abstraction onto themselves. A lot of people in the computer engineering space have no trouble working with alien abstractions for hours at a time, but it's an extremely self-selecting group. We are often at risk of believing that what is normal for us should feel normal for everybody.
The world is bigger and more interesting than screens and screens of uninterrupted plaintext.