You HAVE to present to people a tangible long term issue with using closed-source software that they can UNDERSTAND. Geek ideology isn't enough, and if that's all that you've got, then no wonder closed-source tech is still going to dominate.
I agree completely, and I would like to point out one area that open source can make some inroads in: file formats. Many users of closed-source software use applications that store their data in proprietary file formats. While this may not be a serious problem for users using de-facto standard tools such as Microsoft Office where finding a copy (even an older version) is not difficult and where competitors have created tools that are mostly compatible with these file formats, this is a problem for users who are using less-popular proprietary software applications, where conversion tools may be hard to find or even non-existent. This may be a serious problem if the user decides to upgrade his or her computer and/or switch platforms, or if the user plans on storing those files for long period of time. From proprietary email archive formats, to the often-mentioned scenario of a small business using a database application written in 1994 by a software company that went out of business in 1997, and many other cases, it's a common problem.
While emulators are helpful in such cases, it would be nice if a user in a similar situation had an up-to-date, cross-platform tool that can handle the file. Had the user used an open-source product, there is a higher chance that somebody might have made a tool that handles that file format; the file format specification is available, the source code of the reference implementation is also available, and there were probably other users of the tool, including the developers of it. If a tool is not available, the user could even make a donation to help a technically inclined person write a conversion tool; the open source nature of the tool makes it possible. With a closed file format, however, the user is out of luck in this situation; even if he or she paid a technically inclined person to write a tool, the technical person would have to reverse engineer the file format, which may be very difficult and may be prohibitively expensive for the user (those files would need to be very important for a user to take this route).
This is one case where I believe users can benefit from using open-source software, although I do know that proprietary software products sometimes use open file formats; this is more of an issue of open file formats than open source software.