I'm afraid that nowadays, many successful bands have ONLY been heard in MP3 format... But if it's good enough to make someone's career, it ought to be good enough to judge an arranger by.
I think there's altogether too much residual derision of MP3's left over from the early days, when they didn't sound very good, and tended to be encoded at pretty low bitrates to allow streaming on dialup and early DSL. Nowadays, the state of the art is MUCH higher. To be honest, I listen to most of my CD collection in iTunes, ripped at 192 or 256kbps, and I don't miss much at all...
So, essentially, if the recording of the arranger is a great one in the first place (and that's often where half the problem comes from), and it is encoded on a quality encoder at moderately high bitrates, and than played back on something other than crappy computer speakers (that's where the other half of the problem comes from), it's tough to tell the original from the MP3.
To be honest, most of what I've posted sounds virtually IDENTICAL to the real thing. That's how good things have got. Certainly close enough that I can make an informed opinion about an arranger without having to hear one live.
But ONLY if the two halves are dealt with. A GREAT recording, through GREAT speakers, and it doesn't really make much difference if it is live or a high quality MP3. A billion iTunes downloads can't all be wrong! If you are having problems liking high quality MP3's, I'd take a MUCH closer look at your speakers, then your soundcard, than worrying whether good demos aren't up to snuff...
Remember, this isn't an acoustic instrument we are talking about. At best, an arranger has 16bit samples, so you've lost a large percentage of the dynamic range right there... And if an MP3 can make an acceptable playback of a commercial CD (these DO tend to be well recorded, though
) it should have no problem with the even LESS dynamics of an arranger.
I'll gladly post up a .wav and a good MP3, and challenge you to tell them apart in blind listening test any day. Bet you can't get it right 100% of the time