So basically you think of dualboxing as paying for a mobile potion vending machine. Where have I heard that before....oh yeah, microtransaction MMOs. Well, at least the multiboxer gets to pay twice that of a single boxer. I feel thats a fair price to pay, and a fair risk to take... Just remember to use 2 actual keyboards. SE has a very strict "No 3rd party macros"(BOTS) policy and banned many multiboxers in FFXI for using keystroke emulators. Was pretty sad when they banned a JP friend of mine for dualboxing, though he just releveled a new NIN....in under a month.
I sense a certain dislike of dualboxers from you, why I don't know.
I agree to a certain extent when it comes to bots, like claiming NMs and the such, however I don't find it that big a deal when it comes to 3rd party macros that would let you throw out cures to other party members or yourself while soloing. But I digress and honestly don't care about what moral repercussions that may be involved in such a thing. Those that can will use it and yes they do risk the chance of being banned.
Also, for me it's more than just "paying for a mobile potion vending machine". It's having a second character to help make money as a previous poster said. It's also having a second character for events or missions when you are lacking that extra person. I've had situations in the past in XI where I was doing something that required a certain amount of people but we were lacking someone and had a dualboxer step up to the plate to make whatever we were doing happen.