I have had two kids go through half-day kindergarten, and I think they learn plenty. Our school schedules 2 hours and 45 minutes. It isn't a lot of time, but they spend most of it learning. All day kindergarten uses up a lot of time for breaks, lunch, recess...and lining up to go to lunch, recess, you see my point. As a point of comparison, the kids at our school in half-day test higher on proficiency tests than the all-day kids do. I'm not trying to put all-day down, just saying that longer isn't necessarily better.
Personally, I find afternoon kindergarten to be a PITA. It really chops your day up, you're up against the start time in the am then you have to be home for dismissal. My kids ride the bus, whether you are am or pm depends on the bus route.
But I don't have a problem with it being only half a day, for the reasons I cited plus the others given here.