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For mjpeg_server and web_video_server you are right, here using a compressed input can help reducing CPU usage as the nodes would no longer have to compress the stream internally before streaming to the browser.

However, in general this is not the case for nodes doing image processing. For example the transformation node internally needs the raw image data in order for performing the image transformation (E. g. see https://github.com/ros-perception/image_pipeline/blob/indigo/image_rotate/src/nodelet/image_rotate_nodelet.cpp#L190). I e. if you would require it to use compressed streams and input and output it would have to decompress at the input and compress again at the output --> likely much higher CPU usage for a slightly reduced data traffic. And it comes worth: Think of an image processing pipeline --> each processing step element would have to decompress at beginning and compress when the image leaves. Dramatic CPU usage and moveover the image quality does not decrease ones but step by step decreases while passing through the chain.

Hence, most of the times it is better to use the uncompressed stream for processing chains and only compress the image ones, directly before passing it through the network.