Magic scale

[MaxWilson]

Yes, they can inconvenience the defending nation, but the attacking nation can only move one province per turn anyway. If a barbarian horde replaces a very weak PD (say 6 or so) I can see how you'd view it as a small advantage, but it also means you can't reinforce that province easily with more PD or regular troops, and as you point out yourself it can disrupt the flow of reinforcements. Overall net negative for the defender except in the quite early game where 120+ barbarians still look threatening.

-Max