...is additively blending a set of offset images.
not additive, it's an average. It's just a blur with a low sample count, suitable for real time. This is how current games do it (CoD4, Bioshock, Far Cry 2, etc etc).
if the blur value goes too high, you get shadowing, and if it goes too low, you're sampling the same pixel more than once, which is inefficient. A true blur has to take in to account every single pixel over a range.
This *is* a true blur. It's just low quality. Also, Motion Blur at size 25 is *exactly* like Horizontal and Vertical blur, since I modified them (normalized the kernel and introduced a factor). Only now you can smoothly go back to focus... or push it to a larger size, risking it looking bad. Designer's call.
I'll be using it lots, at least. Already managed to cut down on assets on rocket-days by dropping the preblurred and decolored hills and replacing them with instances of the normal assets with shaders.
Do try out the Motion Blur demo. shows how it looks, and it's gooood, specially the pulsating text below: