I don't see how defending a base is griefing.
When I think of griefing I think about the bastard who pulls Ash on me because my team is killing smugglers. When I think of griefing I think of someone who is camping the director/TIM and deleting items off of the spawn that he doesn't need/can't sell because he thinks that I was rude to him in some way.
NW tower battles are war!! This isn't the arena where you're trying to prove who is the better pvp'er (or whatever else people are trying to prove in the arena ) I think anyone who goes into a tower battle without being outside buffed is foolish because someone else there will be. If you know that a guild has lot's of high lvl members who will pul on you because they can't actually attack you then set up a team or two on your attack force for taking out the mobs around the towers and in the area.
I find it amusing that strategy is automatically called griefing when it doesn't work out for you.