There is a famous parable from the 14th century where a friar meets a wandering horseman. “God give you peace!” says the friar. “God take away your alms!” says the horseman. After some sputtering, the horseman explains that without war he does not eat, just like without alms the friar does not eat (and it had been the same back to the 11th or 12th century). When you need men to burn cottages and hold merchants’ feet over a fire until they pay ransom and sack towns and monasteries, or to camp for months in dysentry-infested mud while people shoot at them with guns and crossbows, the men you get are usually not nice and usually not secure.
Forces like town militias or the English musters of the clergy had a different social character, but they did not do most of the war-making. Those towns had work they needed to get back to, and they didn’t have an interest in dying for some prince, and those mass musters got too many badly-equipped, unskilled people (and they had work they needed to be doing too). So my impression is that in the 14th century, you can get those militias or musters out for a campaign or so in their neighbourhood, but as soon as things sputter down to doing horrible things to the peasantry and trying to take towns and castles, or fighting far away, you get this particular mix of nobles and all these kinds of people on the margins of society.
The 16th and 17th centuries are not one of my periods, but I think there were trends for the poor and criminals to become more prominent in armies. But a lot of those poor saw themselves as gentle too! As gentle status was becoming a hereditary, formalized-in-law thing, it became even harder for landlords who had a bad 20 years to sink down to working for a living and sell off the warhorses and the armour.
