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Internal rotation deficits in throwers have received much attention over the years, but more recent research has really demonstrated that we ought to be much more concerned about external rotation deficits in the throwing shoulder (as compared to the non-throwing shoulder).


While adequate internal rotation is an useful indicator of overall movement quality and you certainly can't overlook it, most throwing athletes will have reduced IR measures on the throwing shoulder secondary to humeral retroversion; it's just a bony adaptation, not a pathology.
That said, alignment issues and soft tissue extensibility changes can certainly lead to further reductions in IR that need to be addressed. All things considered, though, you actually need very little shoulder IR ROM to throw a baseball (or anything).


Most of the motion is internal rotation FROM an externally rotated position.
Building on that, when adequate active external rotation isn't present, we may see athletes present with medial elbow pain or anterior/superior shoulder pain as they work to find motion in the wrong places (e.g., elbow valgus, humeral anterior glide at the shoulder, etc.).
Garrison et al published a great study of pitchers in 2012 that demonstrates these findings quite nicely. 
