Once GPL'ed, any derivative must also preserve the freedoms of its users. This doesn't happen with MIT/BSD and that's one of the reasons there is a vibrant ecosystem around the Linux kernel and the GNU userland and nothing comparable around *BSDs.
So you want the freedom to lose your freedom, for a short term benefit. Make no mistake: you are not as free to stop using that proprietary software as you might think. Most proprietary software strongly encourage you to stick with them, and make switching difficult. The best examples I have in mind are operating systems and office suites.
The same could be said about slavery: you give up all your freedoms for the ultimate short term benefit: staying alive. Fortunately, we try to eliminate this horrible choice, by abolishing slavery.
Likewise, abolishing proprietary software might be a good idea. Until then boycott is all we have.
Most proprietary software strongly encourage you to stick
with them, and make switching difficult. The best examples
I have in mind are operating systems and office suites.
Most people aren't as cautious.
The company I work in currently have to buy and use MS Word 2003 because the government agencies it deals with want .doc, period. The sysadmins are forced to use windows because that's what most people use, and they just won't switch. (And we're a tech company, so imagine the others.)
Inertia is enormous. People tend to stick with whatever they started with (Blender vs 3DsMax is a good example). Proprietary formats and non standard APIs make it worse.
There're also plenty of examples of businesses trying to buy as cheap as possible while at the same time trying to sell as expensive as possible. That just proves that buyers should beware.
Also, proprietary software is not equivalent to high switching costs: in fact, it's a rather subjective calculation. If you don't care about keeping your old emails, switching your E-mail client has nearly no costs, for example. Additionally, "free" software doesn't imply low switching costs: develop a large application using the KDE or GNOME APIs, and you'll find that switching is rather expensive.
In general, your argument is a red herring: Since switching costs may establish a trap, you should warn about software with high switching costs, not about proprietary software.
Actually, most people don't know a thing about the issues around proprietary and free software. If they did, I am confident that most (more than 50%) would rather buy free software.
Now of course those who sell proprietary software happily buy proprietary software: they often don't have a choice. They can't use GPL software. They can however, use BSD licensed software. And I bet they prefer that over proprietary products.