Just an aside here about the overuse issue. I see single sport overuse as an issue for sure (if you are not properly managing your athletes) but my biggest concern is injuries from other sports. I have/had five 3-sport varsity athletes on my high school team the last two years and ALL of them have developed/suffered serious injuries that they have had to manage: 1 ACL tear, 2 muscle compartment syndromes, 1 severely sprained ankle and 1 ligament damage (to both knees). None of these injuries occurred playing softball, but occurred in the fall or winter or as a cumulative effect of playing a varsity sport for nine months straight.
When people tout the multiple-sport philosophy they only mention the benefits (which there are many) but everything has a trade off. Some of these injuries will need to be managed for the rest of these girls lives. A three sport varsity athlete will essentially be working two or more hours per day six days per week from the beginning of august until the middle of may. The toll it takes on these girls' body is tremendous. So, overuse is just a small subset of a larger injury issue: longer, more intense practices and games with little downtime between sessions/seasons.
This is less of an issue at younger age levels and lower high school levels. But, the varsity high school athlete who plays there sports is at serious risk of injury.