A friend of mine recently came back from a trip to Thailand where he was suckered into buying about 30 pirated DVDs on the streets.
The quality of the end products varied drastically. A couple of them were exact copies of the original retail discs and were nearly indistinguishable from the originals aside from the obviously inkjet-printed label and purple dye on the bottom of the disc. These played fine on all of our players and looked great.
Others were obviously foreign-market releases that had Thai menus and stereo-only english tracks but played fine on my region 1 player.
Others still seemed like domestic releases but had no special features, audio options, or soundtrack choices. I think these were ripped from the originals using software like DVD-Shrink. When compared against my retail copies of these discs, there was a noticable decrease in bandwidth used by the audio and video. Where my original had a disc average bandwidth of ~8Mbps, the pirated discs averaged ~5-6Mbps. This is most likely because they were re-compressed when they were ripped, losing information along the way because MPEG-2 is a lossy format.
There were even a couple of discs that were obvious handheld camera bootlegs or half-resolution DivX compressed copies of the movies. These were nearly unwatchable, had no menus, and averaged below 2Mbps when tested.
As you can see, the biggest issue here is quality control. You have no idea what you're getting when you buy one of these discs until you pop it into your player. It may be just as good as the real thing, but chances are you'll be disappointed with what you get.
Now, I've never been a fan of the RIAA/MPAA, but I DO care about quality. I think we are better off with somebody regulating the quality of DVD releases.
Also, the MPAA is the lesser of the two evils, considering that I go to a store and see a $25 music CD and a $25 DVD movie, I know that there's more value behind the 3hr movie than the 1hr audio recording.