As ehra stated, I don't have any good reason to depict Mohammed, but I'm positive that there are instances where someone would have a reason to. For example, what if someone wanted to do a documentary on Muslim history? They shouldn't have to gingerly skirt around any depiction of the central figure of Muslim history just because if they do, they'll be killed by extremists, especially since the documentary would be aimed at a general audience and not specifically toward Muslims.
Many Christians were horribly offended by Scorsese's The Last Temptation of Christ. That movie has elements in it that clearly offend fundamentalist Christian sensibilities, and yet the movie is considered a classic (It's one of my personal favorite movies, in fact). In a world like the one these extremists would like to live in, that movie would never have been made, and we would be deprived of a work of art.