Although your idea has merit, I have always been opposed to releasing client data (especially passwords, hashes etc.) to outside of the control of my company and pentest team.
Just my 2 cents, but I wouldn't trust anyone outside of my NDA scope with that kind of data. So, although I would like to use an online cracker like you propose, from a business sense, I couldn't do it.
But, I'm just 1 scenario. I'm sure there are others out there that would find this capability very useful.