FWIW, we didn't allow our LHP son to play year round ball until sophomore yr in HS. He thought we were being overly cautious/insert nasty-slang-term here. We knew his physique/velocity was not 'projectable' D1 level and, fortunately, we, the parents, didn't have to say one thing b/c experience in Atlanta as rising junior eventually made it clear. (Whew!) That said, he had serious looks by our local high D2 school. He will be playing high D3 next year (assuming a good fall work-out). Though not a brilliant student, he received a LOT of merit aid (the best kind there is!). I think it worked out beautifully. He didn't flame out in middle school. He didn't flame out in high school. And he gets a chance to play ball in college.
Why didn't we let him play year round? My better half knows Andrews and when our kid was 11-ish was told the stats about year-rounders. Yes, there are always outliers but the chances of injury skyrocket with year-round play. Also, re specialization: my player and his buddies didn't do multiple high school team sports but do a LOT of physical activity: wake boarding, snowboarding, scuba, ultimate frisbee, etc. They just came back from their 4th 5+hr day hike in our local mtns.
The small, small, SMALL percentage of players going on to play high-level D1ball (and further) are, in my mind, the outliers. The bulk of our kids are regular kids with a cool chance to play a fun sport in college. Our job as parents is to remember that.