One frequent justification you see for the use of DRM with copyrighted files is the so-called "Speed Bump scenario", which Derek Slater discusses here (Technology as Speed Bump). The basic idea is that, although DRM won't stop filesharing, it is useful in slowing the initial dispersion of copyrighted files as the number of initial uploaders will be reduced. The idea seems to make a lot of sense initially, but ultimately is not such a good idea.
First, for DRM to be at all effective, it will have to be backed up with something like the DMCA anti-circumvention provisions. Suffice to say that the DMCA is extremely problematic. Odd that such a draconian law is needed to justify a mere "speed bump." The words "sledgehammer" and "fly" come to mind.
Second, there is a serious issue regarding how effective the speed bump will be. Yes, you might reduce the number of initial uploaders. However, the spread of a file through P2P filesharing is exponential. What this means is that you have to think of effectiveness in terms of exponential generations. Let's say, for example, that without DRM there will be 256 initial filesharers. With DRM you manage to reduce the number of initial filesharers to 8. This would be an amazing reduction in the number of initial filesharers, only 1/32 of the number without DRM. However, assuming that the exponent of distribution is 2, you've only delayed the spread of the file by 6 generations. Even if the length of time for each generation is 4 hours, you've only slowed the distribution a single day. Whoop-de-do.
Once you are past the "speed bump" delay, you'd better have other ways to deal with files already in distribution. If you don't you might as well give up. In any case, you have to really wonder if all the DRM effort is really worth such short delays.
Third, there is another issue that creates serious cost/benefit issue. The problem with DRM as a speed bump is that it doesn't go away. Even if DRM is effective in the short term (which I think unlikely), its costs are long term. Long after DRM has provided whatever "speed bump" effect it can, consumers are still inhibited from many perfectly legitimate uses of a work. Indeed, many of the costs of DRM are backloaded. DRM likely doesn't create much of an initial issue for many. However, down the line, when people purchase new PCs or devices, DRM is likely to make transfers from old to new devices more difficult or impossible. Looked at from a long term perspective, DRM seems an extremely poor choice if all you're interested in is short term benefit.
Speed bumps make sense on some streets and parking lots. It doesn't make sense to attach them to your car.