Because you don't currently get XP from the mobs if you are dead, even if you participated in killing them. You get the useless nethershards, yes, but not XP. And since the whole invasion XP gain is currently based on killing shit after the latest hotfix, this seems a bit lame since you are dead 25 hours a day when horde outnumbers alliance 9 to 1 and alliance only has two events they can do safely.