If you ask a hundred different people why they use open source software, you're likely to receive at least fifty very different reasons. The responses might range from the ethical and philosophical to stark bottom line financial reasons. If you tell a hundred different people that don't use open source software why they should, using the responses given by the first group, it's likely that most won't immediately, aggressively, start seeking out open source alternatives to the software they use on a daily basis. There might be an interest, but not a pressing one. There's likely to be a healthy percentage of that second group who are just outright puzzled by the reasons given. It's not simply the philosophical ideas that puzzle them -- I've found that those who don't use open source software, or who haven't yet explored the ways free as in speech interacts and relates with free as in beer, the financial reasons are just as ambiguous.
A good number of people (and organizations) using open source software are quite happy with the price tag (or lack thereof) but find the idea of open code inconsequential ( We don't have programmers, we'll never modify this ). The money saved comes from the low cost of the software, and if it should no longer meet the company's criteria, it's time to choose another application.
This perception is selling open source software short. Matt Asay at CNet has a good write up about this idea, and one I think can be taken even one step further.