Everyone knows why (some) publishers use DRM: they’re afraid of people stealing their content and hence costing them revenue. It turns out, though, that DRM can itself have that effect.
I had a pair of Garmin GPS units, one for driving and one for hiking and biking. I decided to upgrade the latter; I wanted a unit with a more sensitive receiver and with a higher-resolution color screen. Naturally, I bought another Garmin, since over the years I had purchased a fair number of Garmin maps I wanted to reuse.
With another trip coming up and with better topographic maps available, I decided to buy a new 1:24000 one covering the area where we’d be hiking. It turns out, though, that many Garmin maps are locked to a specific unit; if you replace your GPS, you have to buy new maps. But that destroys the lock-in — someone who owns locked maps has absolutely no incentive to stay with Garmin. They’re hurting their own future market for GPS receivers.
A similar phenomenon applies to e-book readers. My comments in January notwithstanding, I did buy one recently. (I may write about that some other time; briefly, prices had dropped enough that I was willing to spend the money on something I might not use long-term.) Thus far, however, I’ve confined myself to public domain books (thank you, Project Gutenberg) and library books. I have not bought any e-books. Why not? They’re all locked with some DRM scheme, and there are just too many scenarios that would cause me to lose access to books I purchase. In short, the presence of DRM has inhibited me from buying e-books. (Pricing is another issue. While I don’t expect e-books to be signficantly cheaper than hard-copy editions, I also don’t expect them to be more expensive. That, however, is what I’ve often seen.)
Some of this, of course, is fixable. Garmin could, perhaps, charge a modest fee to retarget a map to a different receiver. Publishers could deposit "unlock" keys and software with an escrow agent people would expect to be around in 40 or 50 years. (I do have some books in my house that (a) are that old, and (b) I reread on occasion. I was quite amused to find that some of them have moved into the public domain and are freely available online.) Even these are inconvenient, and hence will cause some people to refrain from purchasing the items. And that’s the bottom line: DRM may (or may not) prevent piracy and hence boost sales, but it can also cost sales.