What in particular about the rewritten filename would be detrimental to image search traffic?

The filter retains the original filename as well as the alt text.

Given two image filenames: "pogo-sticks.png" and "ic.HASH.x,pogo-sticks,p.png", I'm willing to bet the former will rank higher and lead to more pogo stick traffic and sales because of the straightforward filename.

From the horse's mouth: "optimizing your image filenames and alt text makes it easier for image search projects like Google Image Search to better understand your images."

http://static.googleusercontent.com/external_content/untrust...

Anything that makes it harder for Google to understand your site is a losing business proposition.

I'd rather use PageSpeed for Firefox (optimize in dev, set the appropriate filenames, then deploy to production) to avoid any possible ranking penalties caused by this filter in production.

The search engines will spider/cache the resized version, so people looking for a higher resolution version, may skip over your site?

I'm surprised it doesn't convert to (and if possible serve in) WebP, the image format created from WebM.

It was these guys (the "speed" folks) within Google that came up with the idea.

http://code.google.com/speed/webp/

Maybe in a later version?