Because if you eventually want to move on against your neighbor's friend, odds are you want to make sure that this neighbor won't backstab you the moment you do so. If there's the possibility to cripple him for a while, then you can start a war against him first while the other guy's distracted (or not quite as committed as the first target would have been to his friend). If you succeed in putting him down, you then know you have enough time to launch a war against the other guy without having to seriously fear the first guy. If your ambitions are greater than a meager addition of a rural to one's realm, knowing you can incapacitate another is important. Because seriously, gaining a rural region is of little interest to realm-wide prosperity and to ambitious people's agendas. The ones that have the most to gain from small border skirmishes are usually those who don't have a say over whether a war will take place or not.
Will the neighbour's sit idly while you stomp their friend? I doubt it. You will get attacked from two sides instead. Nice theory though.
