The audience for this metric isn't consumers choosing whether to get an iPhone or an Android phone. It's developers choosing whether to support iOS, Android or both. For that audience, this is a very relevant metric. It's also a fairly objective one.

(Of course there's a second-order effect: users will choose a platform partly according to how well it's supported by developers. But anyone choosing which phone to buy now on the basis of this metric is betting on a longer term than they'll probably keep that phone for.)