The reason why we are doing this, as it might be a surprise, is exactly because of World PvP. People get enormous teams and shut down an entire area of the game for hours. We are talking about hundreds of characters. Realistically, multiboxers only do well in Wintergrasp and in the open world. Many get the notion that multiboxers dominate everywhere but the data and statistics show that they don't. The amount of complaints we receive during these hours of slaughter peaks astronomically, but this was never the core issue.
The core issue is that the hardware is getting much much better. It is now possible to run several computers at the same time, with some of those computers having even two processors. This has enabled some players to make 200 character teams and have no issues running this. We have to set a limit somewhere. We took a look at what kind of multiboxing was possible 10 years ago, without running a supercomputer, and we also took a look at most multiboxers' playstyle, character count and what they mostly do and decided based on that.