If a kid is a real athlete, taking off a couple of months for basketball won't matter. Would be just like riding a bike. The trouble is that there are too many girls who are not good athletes who are out there pitching and are making pitching coaches a lot of money.
CoachFP - I think you are missing the point with training during the off season, especially with speciality positions like pitching. Its purpose is two-fold: a) work on new pitches and/or "perfect" existing pitches; and b) maintaining and keeping your pitching at a high-level.
While I think it is important for kids to play multiple sports and to take some time off between seasons; the off-season is a great time to focus on a few core softball skills like pitching, hitting, etc. so the transition is a smooth one once the pre-season starts again. As redhack said, it's also a time for the not as athletic kid to "catch-up" to her peers and stay competitive.
Pitching well is NOT just like riding a bike. Pitching is the most complex activity and movement performed in fastpitch and requires year-round dedication to do it well. Any coach worth his salt can immediately tell which pitcher worked hard in the off-season and which ones didn't pick up a ball for two months.