The fun, and challenge of controlling 2+ characters is the reason most people multibox, I personally 5 box my redneck shamans because I hate waiting for groups, and really love blowing people up in a global cooldown. Heroics are intersting without a tank and make for quite a nice and unique challenge.

Playing someone else's account is bannable, against the ToS and therefore against the forum rules. Do NOT do this you WILL get banned, this is something blizz has been hitting pretty hard since the RAF bonus was added.

However, running multiple players can be quite addicting and rewarding, so stay within the rules and stat up another account...you know you want to :P