Not unfair if everyone can do it.
I don't think account sharing should be against the rules, nor should it be bannable. Jagex should just have a little "warning" in stronghold of security or whatnot that says "do it at your own risk, idiot". If you lose your account/items because you share your account, it's your own damn fault.
While I see your point, account sharing does one other thing. If you have 2 or 3 people playing an account it gives that account a huge advantage over other accounts with only 1 owner, as far as the ability to train is concerned. There would be no way for me to compete with an account that has twice as much available time to train. As others have mentioned about the Firecapes, it can also let someone do something for an account that its original owner might not have been able to. Account sharing can give an unfair advantage.
Also it's unfair that a middle school child can play 8 hours a day, while I can only play a few because of studies. Do you see me complaining?
e: NO, I don't complain, because it doesn't matter if you're better than me.