As Otlecs already mentioned:

1.
I enjoy playing the game when I got time. I don't want to depend on other players to visit an instance, I don't want to spend my whole afternoon in Shattrah spamming "LFG / LFM xxxx". When I get online I want to play. When I need to exit the game I don't want to justify myself in front of the rest of the group why I need to leave - I just want to leave because I'd like to so something in real life, with my friends or my girlfriend instead of hanging around in World of Warcraft.

2.
I got the hardware - why shouldn't I use it? Haven't bought that many stuff especially for multiboxing.

3.
12,99 € / account * 5 = 65 €.
It depends on your income if this is a lot of money or you don't need to think about it. I've got a job, I've got a car, I've got my own home and I've got a solid financial plan. And this plan tells me that I can spend the 65€ for my hobby. I've had photography as my hobby before I started with World of Warcraft - I've spent about 100 - 200 € / month for this hobby so Warcraft is quiet cheap.

4.
it kickz ass running around with 5 toons and one-shot everything that passes your way! :lol: :lol: