Well, there *is* a practical reason, though it might seem paranoid to some
people: Proprietary software often contains malicious features, such as
spyware which records your activities in secret, backdoors which can change
your system at any time, and Digital Restrictions Management, which
artificially restrict what you can do.
Other than that, it's more or less just principle.
But I've found that people understand the merits of freedom in computing:
that everyone deserves the right to control what their computer does, and
that if the users don't control the software, the software will control the
users. (Actually, once, when I was telling some of my second-cousins about
this, they finished what I was saying, word-for-word.) Obviously, most people
will continue to use proprietary software, but convincing people to use only
proprietary software is nowhere near as important as convincing people to pay
attention to the freedom issue.
