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.
