The designer will usually provide very high quality PNG's/PSD's. It's up to the developers to scale it down to the optimal size for each device.

I agree, and as stated in my other comment, I believe it needs downstream support to be adopted properly.

To that end I plan to add support to ImageMagick, and perhaps write a PS plugin.

Pngout, optipng, deflopt, jpegtran, gifsicle, cwebp, zopfli, and so forth are all command line tools.

I'm not really sure why you think that this stuff is meant for designers. Recompressing images is a frontend optimization. It's something you automate. It's a task for machines, not humans.

If you care about jpeg size you're probably already using a build/preprocessing system on your assets. This is just another tool in the chain.