There's an argument in favour of DRM that says, basically, it won't beat the crooks, but it keeps honest people honest. I think it was Cory Doctorow who said that keeping an honest person honest is a goal akin to keeping a tall person tall. But more than that, DRM often fails those honest people too, and what do they do when that happens? If the DRM gets in their way, either they stop being your customer or they stop being honest, breaking the DRM or finding a copy that's already broken.
So, to summarise, this argument says that DRM is imperfect, but it keeps the sheep in line. That's only when it works. When it fails, it keeps honest people out of the purchases they honestly made, and it makes crooks out of customers or, at best, makes non-customers out of customers.
Mokalus of Borg
PS - Remember, kids, it's legal to rip your CDs, but not your DVDs.
PPS - And the DRM is just part of what stops you.