May calving has a bunch of advantages:
- maybe not a lot less labor, but certainly easier
- less stored feed fed, gestation needs a lot less groceries than lactation
- better calf health, less battles
- lactation curve matches the grass growth curve

A couple of downsides:
- breeding cows in hotter weather
- lighter calves or a later calf sale date 

I think the pro's easily outweigh the con's