Yet another indication of how out of touch with their customer base they've gotten?
The key idea of the Macintosh and the NeXT---the personal computer---is no longer foremost. Maybe it's time to swing back that way again.
So this is a panic PR move to stop core content creator loyalists jumping ship. If they were further advanced they'd be previewing stuff, not just bumping the trashcan specs a little.
I'm still not sure this is very developer friendly. Developers are a niche within a niche, although a disproportionately influential one. Developers probably care more about the MBP than the MacPro, and the MBP is where it is.
For the MP - I would guess we'll see previews in the autumn, with shipping early next year.
For apple engineers or for the customers? The words from apple make it seem like the goal is to improve modularity so that apple can make easier changes.