You can do that with some proprietary software too -- proprietary doesn't have to mean no source code and no source code modification, unfortunately it has come to mean that for too many businesses and users willing to pay money to those businesses, so yeah, it's a point in favor of open source. I also appreciate that contributing to and encouraging use of open source has a societal benefit of training my eventual replacements for free, and open source of a particular license guarantees improvements to that software from other parties than the primary maintainer remain open.