Other than that, a self-taught student may be tempted to tackle easy problems. For instance, I don't think I would have ever attempted a ray tracer in c++ if I had learnt from home.
And what about teachers? It's not like wikipedia would say, "hey, looks like you're creating a 2d game. ever thought about this mapping algorithm? it may make your code simpler?" You could get that information from the internet, but you'd have to realise you had a potential problem to do that, and without a teacher, who'd tell you that?
Most of all, a lot of this thought comes from retrospect. Of course I now know what subjects make up a good computer science degree, but before my degree I wouldn't have known the significance of compilers, for example. Nor would I have known where to /start/, and where to go after that.
It's not that it's not possible to be self-taught, but you'd need a hell of a lot of passion and motivation for the subject to shack off all the problems you'd encounter without a teacher aiding and guiding you.