Regions actually recover quite quickly even after you call peasants. Also, all peasants that spawn from a region are based on RC stats of that region. Where do you think these peasants are getting their weapons and armors from. Peasants aren't really those weaklings you used to slaughter anymore.
I've very rarely slaughtered peasants but I have deployed them a few times and the economic impact has been heavy troublesome - especially when they take heavy casualties. I'm currently exploring what can be done with more reactive peasant militias so I'll not venture any further opinion until I have more data to work on.
As for region lords being able to call peasants, I am actually against that. That would make attacking another realm a living hell.
Lords who wanted to take advantage of that would have to be in their regions, and as most realms are struggling with low noble density I suspect that would damage their defences in other ways.
