The problem is that by defending their territory only, they let their neighbors grab juicy Terran Hegemony worlds (with lots of advanced tech on them) cutting that Successor state off from access to high-tech Hegemony components.
It also lets their neighbors build up forces on their border, instead of trying to whittle them down with strategic raids (i.e. hitting a fuel depot before the neighbor's fleet can do its refueling), meaning that neighbor's fleet can now attack deep into their territory, while they have to cover multiple systems where the neighbor could attack.