Exif data is particularly useful for preserving copyright metadata and (optionally) contact info for the photographer. Stripping too much metadata perpetuates the 'orphan works' problem and creates lots of photos floating around the internet that can never be used commercially, because the photographer cannot be identified.

More info here from the US Copyright office - choice quote: "For good faith users, orphan works are a frustration, a liability risk, and a major cause of gridlock in the digital marketplace." http://www.copyright.gov/orphan/

Also, the UK has effectively given everyone the green light to steal photos lacking metadata because it's basically too difficult to find the photographer. http://www.bbc.co.uk/news/technology-22337406

And from my perspective, I release many images CCO Public Domain with my email or name in the metadata, and I'm annoyed that this metadata is not preserved because it means people may be reluctant to re-use my images due to (non-existent) copyright concerns.