Because usually the runner is braindead and aggro's every mob they come across. I was in a Colony Invasion earlier and they'd dragged the boss mob back to the 6th mobs spawn point and aggro'd the monkey for added giggles. That was the 6th, 7th, 8th and 9th spawns of Hirogen plus the monkey that the other 4 were having to deal with whilst the idiot that aggro'd them hid around the corner under stealth. I thought he was trolling but from the incoherent typing in chat I think he was just on day release and high on prozac, either that or came from a very shallow gene pool.

The runners aren't a problem if they're careful not to aggro all the other mobs but that's rarely the case in my experience, and when they're not all they're doing is making the whole experience more painful for everyone else by dragging those aggro'd NPCs back to the rest of the group. Considering many of the NPCs in there are using damage types with limited available resistance (Breen and Cold, Undine and Psionic and the Hirogen with the Tetryon that seems much more effective than mine) those extra mobs make things really annoying for the rest of the group.