CD's are saved as images. ROM's are chips.
Trading something in print and something out of print (2 hours, 20 years, time frame doesn't matter) are completely different things IMHO. Anything out of print is no longer making the property holder money (and making a copy of something that isn't making them money isn't costing them money.) That said, both are illegal and under the Disney copywrite law nothing electronic will be public domain for another 50 years or more.
Emulation is protected under fair use laws (well, what is left of them under the DMCA which isn't much.) If you own the original ROM, then having a copy on your PC for an emulator is fine. Giving a copy of your ROM to someone else is not fine (technically, even if they have an original ROM you still can't give them a copy.) If you follow the law, there is nothing wrong with emulating anything. Some people not following the law doesn't make the entire thing illegal or wrong.
All of that out of the way, there is no reason to pirate Dreamcast games as they are readily available for the most part.