Proprietary software is not illegitimate, just problematic from the economic sense. Open Source software gives many advantages to the end-users and is a generally a good thing. Copyleft licences are important in making sure certain software is not abused. It is not immoral to use proprietary software, it's just risky. Using or producing software that is not 100% open-source but pretty close, can be a good idea, depending on its licence and the general attitude of its developers.