Reinforcements are limited like the money. So for example, when you have $350, and 6 reinforcements, you can either build 6 militias or 5 infantries. 5 infantries will be the better choice.
And when you play in Europe, you will usually have more money than reinforcements.
But even when you have lesser amount of money, militia are still not very good because of their low range. You need to constantly expand or fight your opponents, and you will lose a game if most of your units are almost stationary.
Guerilla Warfare makes their range a bit bigger but even that's lower than the default Infantry's movement range.