Every IT manager worth his salt will know that there comes a time when you need to find out more about your underlying libraries/systems than the API documentation tells you. With closed-source systems this often means scouring the web for anecdotal evidence or begging the vendor for more detailed information. Having the source available is the ultimate documentation and can save you lots of hours of guessing and testing. To then have the possibility to change the underlying system to better suit your needs, legally distribute your changes and even maybe merge your changes back into the mainline distribution, can be invaluable. So yes, I think it's a strong selling point. For clueful people, that is.