I think it needs to be considered that if caricaturing, criticizing, or generalizing Muslims risks legitamizing resentment, hatred, or violence towards innocent people, the Charlie Hebdo massacre makes a pretty good case that it's also possible for criticism of, say, cartoonists to run the risk of legitimizing violence against people with a podium who say things you don't like. After all, someone has to stand up for the "little guy" against those big bullies, right?
...And we're all oh so ready to define the "little guy" as us. We're the outsiders. We're the mavericks. We're the rebels. We're the bullied. What actions could we take against such an entrenched, evil force that would not be justified, given what is arrayed against us?
Amidst all the noise of modern life, it's so damned easy to feel that one has no voice at all, furthering the suggestion that they're justified doing something hideous because, dammit, no one will listen otherwise. That's part of the reason genuinely free speech is important, beyond narrow blinkered ideas that only government can block free speech. It's a bulwark against the idea that only entrenched interests can get heard. Even if some of those things that get heard are offensive or stupid.
There are some ideas, and some ideals, that are important to uphold. It is important to believe that adults, given access to others' ideas and information, can pick the good ones from the bad ones, find information beyond what supports what they already wish to believe, what is comfortable to believe. That they will take the trouble and do the work in order to do what is right.
Every day, we see plenty of examples that fly in the face of that ideal. Yet it is still important to believe in it, even if the world we see around us often doesn't seem to live up to that ideal. That democracy, if imperfect, still works enough that we don't just give in and let a plutocracy or a theocracy or some other form of oligarchy rule our lives.