The issue from a PR perpsective is that MS don't seem to be controlling the story here. Will WP8 apps work on 7.8? Sources are totally conflicted. Will 7.8 do multitasking? No-one knows. MS should have been all over this out of the gate- explaining exactly what 7.8 users are getting. Instead everyone is resorting to rumour and assuming the worst.
It sounds like they're doing just this in spirit by backporting some features to Windows 7, but calling it Windows 7.8 instead of Windows 8. Marketing mistake?
The Lumia 900 came with WP 7.5. And now it's not getting WP8, just some features from it. How is that the same?
EX: iOS 6 comes out this fall and is backward compatible all the way to 3GS phones which came out June 19, 2009, but not 3G which came out July 11, 2008 which they stopped supporting with iOS 4.2.1 on November 22, 2010.
However, you can buy WP7 phones right now that will not be able to run the latest OS when WP8 comes out. If I were selling these things (or Nokia), I'd be pretty frustrated about that.
Why pre-announce? Osbourne effect FTL.
I don't know if this will die down or not; that's what eventually happened when Google revealed the Nexus 1 (iirc) wouldn't be upgradable to 4.0. So far, though, this point is significantly distracting from the amazing improvements in '8 - note how the linked article mentions it in the first paragraph, and I've seen at least one mainstream article [0] focused entirely on it.
There must be a lesson about product launches in this. If Microsoft had a good answer for this from the start - by "good", I mean one that satisfied journalists and their readers, and/or made it hard for anyone to stoke outrage - the positive press coverage for WP8 wouldn't have been diluted by this.
[0] http://mashable.com/2012/06/20/no-upgrade-windows-phone-8/