In most games I would agree, but in the case of this type of game (1 vs 1 global domination) the plot as flimsy as it may be is as good a plot as any.
You have the classic clash of good versus evil/ right vs wrong/ my way vs your way of running things. In this game PL is the bad guy or the initial aggressor, so they should and do have the initial advantage of knowing exactly where they want to attack and be able to adequately get reserves and what not to resupply their borders.
FNU on the other hand already has the land advantage considering neutral territories never attack and make a perfect unguarded line. Hence since FNU has no idea who the enemy is initially as is the case with most treasonous acts, it makes perfect sense to me that FNU should have to either place units on all borders just in case, or take a chance and leave a few borders unguarded....
Worse case scenario, PL invades your weak side on the first turn and you get the addition of guerillas to stall the invasion while your defenses get repositioned and force the PL retreat.
Another point is to place your units on the border of the joining land that has the larger capitols. ie: your on capitol (8) and you have a capitol (8) and two capital (4) territories touching your country. Ignore the smaller capitol borders as anything coming from there will not have enough strength to matter much anyway.
Like you said before though, when playing FNU on a small map with a poor layout, you have to think things through more thoroughly.