Its an interesting theory Xzin, however its been done with Bots for the last year or so. Admitedly, there are probably only a handful of farms that do this worldwide. The only real costs are paying the developer to write the software, buying sufficient equipment to run it on (With stripped down clients with no graphical rendering you can get 30+ bots running on a Quadcore) and paying someone to 'monitor' the bots and then generate gold from the auction house.

I can forsee a problem if Farmers do begin to use Multiboxing as a viable method of gold creation, however the threat from bots is far greater and most likely far more viable from an owner/operators point of view.

No I'm not talking about the bots you see running around Tanaris farming :P

The level of botting that is currently possible can consist of stripped down clients, fully automated groups, with Excellent Warden protection operating entirely in instances, automation of repairs, mailing, even Raid tactics. These botters do exist, however its highly unlikely you would ever see them, and if you did, theres virtually no chance you would realise they are bots because of how human their movement can now be made.