Just from my point of view (and probably full of holes, but I tried to get the community thing across):
At a high level perspective, getting a CD-R, or tape trading is NOT the same as MP3 downloading, and I'll tell you why....
As lurch pointed out, tape trading has been going on forever and a day. Tapes have now simply been replaced by CD-Rs as a better medium. Back in the day, tapes represented entire albums, or better still, compilations made personally for one another. It was a way of getting your friends into the same music as you, and often you would trade when you saw people at shows. Even at ProgPower this year, people came down with tapes and CD-Rs to hand to friends as a way of showing them what they liked lately. This is very cool in my book.
For the most part, the difference with these trades is that TWO (or more) parties need to personally know each other in order for the trade to occur. This is actually quite healthy for the underground because it promotes community (yes, community is vital), and some thought/discussion has gone into the trade beforehand. Also, people BUY blank CD-Rs, they BUY padded envelopes, and they BUY stamps in order to send the CD-Rs to each other -- sometimes overseas. There is cost associated with doing this.
If I like a CD-R, I will buy the album. Very simple. I will then probably share the CD-R with someone else, hoping they might also like it (why keep original + CD-R?). Any CD-Rs I don't like, I'll pass onto someone else, or let them collect dust. All this also applies to promos.
As you've seen, the guys here exchanging CD-Rs are using them like promos. We're all doing write up reviews for them in seperate threads, albeit brief, but sometimes it might spur someone else to look into an album. Again, this is what we do with promos also, remembering that this is a webzine forum trying to support the scene with writers who buy music, get free promos, share CD-Rs, and pay $$ for the existence of a site and forum. This is all supportive of the scene.
Now onto MP3s....
MP3s are downloaded for free -- no medium (other than your pre-existing hard disk), no mailing cost, no envelope, etc. They are downloaded anonymously and listening thoughts are kept private. You don't care where the MP3s reside, or who originally ripped them. It's just a massive core dump on to your hard drive, and there they sit until you decide to delete them. It doesn't promote community trading which is beneficial to the underground. It does not support the scene as far as I can tell....
....unless you're buying albums based on MP3s that you like (as one would do with the CD-R). Of course that changes everything. The debate in this thread has discussed the harm in people only downloading MP3s because they're too cheap to buy the real thing (not saying anyone here does that, but these people do exist nevertheless). This means that the scene is supported by ONLY those who BUY albums, and the freedownloaders are riding their coattails.
You don't need to know anyone in order to get a free MP3. You don't need to have a webzine. You don't need to write reviews. You don't need an alliance with a record label, etc. It's all anonymous and underhanded if you only download free music and never buy anything. That is fucked up!
Of course there are permutations to all this, but I tried to squeeze it into a nutshell.