I began multiboxing somewhere during the TBC. I originally got into multiboxing searching for different challenges. And boy, it was satisfying (not in a sexual way) to clear just your basic 5 man instances alone and smoothly. I consider myself to be (or rather, I were) a quite competent player. We were not fighting for World or even EU firsts, but we got quite a few server firsts during the Classic times. After Classic I pretty much saw all the content up to Cataclysm.
In my mind the economic issues related to the professions is the most valid concern regarding multiboxing, excluding some extreme cases of 40 character boxers...
Bank space. lvl1 alts, maybe? Ok, there is a 10 player limit per account but I never had issues with my bank slots (and yes, I'm a hoarder. 52x stacks of thorium bars anyone? Etc etc).
Unfair advantage in PVP. I tried some multiboxing @ lvl 70 arenas and I got seriously humiliated. Granted, I never was a PVP guy and I did not have a change to try how 4/5x Shamans blow people up during Classic times. However, I can confirm that melee classes really f*ck you up and I've seen only a handful of multiboxers who actually could keep their characters in check during the hectic (arena) PVP. Multiboxing in AV was quite fun and easy but still not a game changer. A bad multiboxer can really mess up WSG and AB as he is taking 1/2 or 1/3 or the group so maybe there should be some kind of a solution for this? Also, if one of your character dies in a BG, it's pretty much dead in the water...
Also, I really cannot grasp the complaining regarding 5v1 World PVP. Would you alone attack a 5 man party going to an instance? Your un-friendly multiboxer is ruining your herb/ore/whatever farming? I would claim that your farming can also be disturbed enough by one (competent) player. T2 dude came and oneshotted your miner/herbalism alt in Un'goro, what would you do after a couple of corpseruns? 1) continue to compete with him of the Thorium spawns or 2) Log off/switch character? I chose nr. 2, always.
It seems the stance towards the multiboxing is positive from the Devs/Twinstar. However, if they decided to ban it, all this 1 account/1 IP mumbojumbo is unnecessary... It would only take 1 report and you would be gone. How could you survive and continue your multiboxing in any meaningful way on a 1x server?
Just my two cents on the matter..