@ steeler's fan:
Many would agree with you that multi-sport athletes are preferable. sbfamily posted a link to a study paper in the last few years that suggested college coaches prefer multi-sport HS athletes when recruiting (though I read that study and had some serious concerns about its methodology). Our head coach feels quite strongly that multi-sport athletes should not be penalized in travel ball and that they often are preferable to the one-sport player. My agreement with him is limited to the burn-out and overuse injury issues. Concentrating just on softball can lead to softball burn-out and muscle imbalance/overuse injuries. But concentrating just on softball can also lead to a better set of softball skills and can eliminate the kind of travel ball conflicts that Klump has described. I think, though I don't feel uber strongly, that I come down on the side of choosing one sport by the freshman or sophomore year and then making sure that a physical conditioning program is in place that protects against muscle imbalance and a training program is in place that allows for some off-season vacations from softball.
But generalities have to give way to the consideration of any particular athlete's situation. For talented athletes at small schools, for example, there is pressure to play multiple sports, and quite frankly HS softball programs benefit from the participation of multi-sport athletes who don't play travel ball. And if the young lady doesn't have aspirations to play sports in college, then much of the rationale for playing only one sport goes away.
If the young lady has a passion for softball that exceeds her passion for other sports, I think I would advise her to become, in her freshman or sophomore year, a one-sport athlete who takes steps to condition her body to avoid muscle imbalance and those injuries that are common to softball and who takes further steps to give herself some limited mental and physical vacations from the sport.