First I would opine that the (what I regard as a perjorative) term of labeling early militia building as "spamming" is unwarranted.
As both armies historically had to draw initially on such a micropscopically small regular US Army (the CSA from those ranks of the recently resigned) it was imperative for both sides to draw upon large bodies of volunteers and recruits to bulk up their military fast and early. It is historically what happened en masse on both sides. And these troops are known as militia.
Johnnycai has done a good job spelling out the benefits and drawbacks. Like him, I will rarely build any other bulk infantry in the early years thru 1863. Using ala carte artillery and some cavalry, you can cost effectively build up your army quickly and inexpensively in comparison to the preformed regular infantry brigades. In addition to the advantages he spelled out there is a cost savings in dollars. About 20+ % as I calculated it, allowing you to build larger armies. And even with the new 1.14 War Supply cost addition of 1 per unit coming soon, militia is still a bargain.
The militia growth chain starts with militia, graduates to conscipt and then to 10-16 line infantry or directly 11-20 line infantry. Units can be built quickly by adding ala carte artillery. The Union has three conscipt "graduating" leaders in Halleck, Siegal and McClellan that when utilized in different theatres can help quicken your army building pace. Additionally there are militiamen like Couch and Sherman who add 10 cohesion points in battle to militia when you choose to fight with some in their virgin state.
Additionally there are generous amounts of free militia replacement points granted in the early war to both sides. Now that I use militia alot, I have found myself actually running out of them and having to build a few more.
If you're wondering how your opponent may be building bulk faster than you, look first to his building militia. It's a nice historically accurate touch to the game as well as a good army building tool.
Sky