No doubt, it´s a powerful tool, since it´s all about emotions, to listen to some that we like. And we learn a lot when we work with emotions, don´t we?
I might not expressed myself well, it went in a misleading way...
What I meant is that depending on the kind of rap (or even pop, modern rock, etc) it can be an hidrance to the learning, mainly to the ones that are in the very beggining of their studies. Wether we think of the too slangy ways, wether we think of diction (sometimes) or the machine-gun singing (wich isn´t easy to a begginer).
Other than that, music is a gratifying experience, and it´s even recommended, so why not in learning or practicing? regardless of the genre.
I would just advise that one should try the ones that suits your listening skills first.
By the way, music should be also a good way to hone one´s critical thinking, because there are things in music that go beyond reality, etc.