Every topic about multiboxing will end up the same way. Single char user saying boxers ruin gameplay, boxers saying it doesn't.
Now please lets be honest in here for a change. Boxers actually do affect the entire outcome, depending on what we're doing. I think, for example, horde boxer totally ruin the gameplay for horde in a battleground, and ruin the gameplay for alliance in WG. It's been proven several times, horde lose WG 95% of the time (and yes, getting gy farmed, I've been there god knows how many times) when there's no boxer around, and win most of when they have few. Thankfully, they contribute a lot for the server, so as for me, it's way better to keep that way. And as for me, the main trick is to avoid them at all costs, or gather a strong group, whatever...