The biggest difference in Mame and MP3's is that for the most part people aren't downloading 20 year old songs they are downloading songs that are available at every music store in the country and thus not buying those disks. If mame started emulating current arcade hits you would probably see the arcade industry get tougher.
The OTHER difference between MAME and mp3s is that people DO download 20 year old songs, BUT those twenty year old songs are largely still available new in the store. While 20 year old arcade games are almost totally unavailable new (Ok, you can buy a few, but not many).
But, on the flip side, music can be recorded quite legally due to the home recording act of whenever that was, but on the flip side there is NO rom recording act whatsoever.
ALso, as others have said, most of the game copyright holders are completely unwilling to release any versions of the game that have the functionality of the Mame versions. Yeah, sure you can play Pac-Man on your gamecube using an Analog controller, and the wrong screen orientation. Whatever, I am not paying for that, not when MAME works so much better.
Here is what I would like to see.
You SHOULD be able to buy the roms. Download or CDROM, does not matter. What does matter is the license you get. I would like to see it where each rom you buy comes with a serial number sticker, which you can place on your cabinet, or computer case (or wherever). That way all those operators that are still putting classics out there can stop repairing them all the time, and just start using more reliable PCs instead.