Besides the fact that 1080p is common and marketable, it would be the perfect resolution to mirror to an AppleTV.
And having both the 4.7 & 5.5 phones be @3x would motivate developers more to add @3x assets. If the 5.5 phone is the only @3x device in Apple's lineup for a while, it might be tempting not to invest.
And then a 4K (3840x2160) at @3x might be perfect for a 12.x inch iPad. That would mirror perfectly to 1080p or 4K TVs. It would be 16x9, which is hard to imagine, but mirroring to projectors might be more important for a 12" iPad than the smaller ones? I could really see it used in presentations and education.
I say go with the standard resolutions when in doubt, even though there is no real Apple precedent.
(edit to add content)