A search of Dr. Math turns up nothing. I suspect that it's just something some math teacher teaches for the sake of teaching it, not because it's at all useful. But I'm not going to go out on a limb and say it's not useful, just that I can't think of any use for it. All I know is that I got through differential and integral calculus and differential equations and multivariate calculus classes in college without ever hearing of the concept of a semi-prime number. I never did take a numbers theory class, and if it is important, that's probably where it would turn up. I have to wonder what grade this kids is in who's studying it.

One definition is found here:

http://www.mcn.net/~jimloy/semiprim.html