Do we really want to support an ecosystem that relies on an identifier so easily controlled and identifiable as a phone number? Building systems around legacy systems such as PSTN, and newer proprietary ones such as Gapps may be in Moxie's words, "almost a sure recipe for a successful consumer product", but it also increases the number of actors you need to trust therefore making security more complex.
https://news.ycombinator.com/item?id=11672892