I get that one constantly. Most people can't use simple logic, I don't bother to try to explain it to them because they either won't understand or they simply don't want to be convinced. Let's look at the facts:

1. It takes less time to multibox a group to 70 than to level a single character, if you do it right. Even if you're slow, you can get all 5 to 70 long before mr. single boxer could get his second toon to level 30.
2. Gearing up for PVP takes the exact same amount of time as a single character because they all gain honor/arena points at the same time
3. Grinding gold is much, much faster. You can do all dailies in a group, and in about 1/3 less time than a single player, you end up with 4-5 times more gold.
4. Instances and Heroics are much faster as well - you never have to look for a group, there is only one point of failure (you), and you can develop a system and run it every time instead of having to deal with random idiots who can't work together.

Seems to me like multiboxers have even more time for real life. How much you decide to play is entirely up to you, but given the above, a boxer can gear up 5 characters faster than you can gear up 1.