The constraints on a geostationary orbit are fairly tight. It's not a wide open orbit, but rather a very narrow line around the earth. The further from dead center, the more fuel they must use for station keeping. They like to stay dead-center because it keeps them in the same place in the sky and reduces the amount of fuel they must expend.

Even dead center, there is drift for several reasons, so fuel must be consumed; just not very much.

The fear is that an enemy will enter the orbit very cleanly, going the opposite direction.

When there are millions of BB's flying at them at 14,000 mph, that's a lot of damage from even one hit. Worse, the entire orbit becomes useless for anybody for many decades.