We’re not doing bodybuilder style training that causes extensive muscle damage or going to CNS failure like a power lifter might. As long as they keep the reps low and don’t go to failure, they’re fine for their easy runs the next couple of days, and completely recovered by their next workout day.
The “Kenyans don’t do it” argument doesn’t hold much water for me. There are a lot of factors at play that make the starting points different.
As for benefits (worth noting I work with high school girls):
1) Develops basic athleticism (possibly more important that aerobic conditioning for girls who show up without prior sports experience)
2) clearly sends the message that female runners need to be strong, not just skinny.
3) increases bone density
4) temporarily increases testosterone
5) strengthens connective tissue and stabilizer muscles (including the core—I think heavy squats are probably a better core workout that most core circuits)
6) Might make them faster by increasing max muscle fiber recruitment