You're saying chrome has a larger attack surface which is true but I doubt it has more exploits(per arbitrary unit of time such as month) than IE, if I'm wrong then I don't mind being corrected.
I'd think you would want to support as many browsers as you can hosting an exchange. I'm not so much sold on Google products these days, they retain a lot of data. Not that I have something to hide, but I do value my privacy and take offense to having it violated. But I know they have to play ball or the NSA will dig out their magic wellness stick.