Advantages:
Discourages gangbangs
Not really. It discourages traveling through allied or friendly lands. (Or, really, the lands of anyone you don't want to piss off.) This is not a requirement for "gangbanging". For example: Caligus and Perdan could both easily wail on Ibladesh without penalty with this proposed "discouragement" simply because one was on the Ibladeshian northern border, and one on the eastern border. They can both individually attack at will without passing through allied land.
Discourages blob armies
That depends. If the amount of damage is a linear figure based on number of troops, then spreading out the troops only spreads out the damage, which
could be desirable or not, depending on the circumstances. Now, if the damage was non-linear, for example Troops^2, that would discourage blobbing
while in your own or friendly realms.
However, this would also
encourage blobbing once you cross into enemy territory. You would want to concentrate damage into one region at a time, so as to maximize the impact. Would you rather do 3 damage each to region A and B for two turns (for a total damage of 12), or 9 damage to A this turn and 9 damage to B next turn (for a total damage of 18)? Also, this would really screw over the defender. You are
encouraging the attacker to blob in order to maximize the damage, but
penalizing the defender for blobbing up to concentrate troops to combat the attacking enemy blob.
Encourages attacking realms closer to your own (well, you're not marching through allied territory if they're your enemy, right?)
Not directly. It encourages you to not march through allied lands. Thus you keep to enemy or rogue lands, which are likely to have crappy roads, slowing down your progress, making coordination difficult with mutli-turn moves, and generally slowing down the pace of the war to a crawl.