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The 50 and 85 are at least 1 1/3 stop faster, depending on which lens you're talking about, so they give more flexibility when it comes to controlling DOF.
Just like any lens, it can be used for a portrait lens but there are better options for some people.
I'm a fan of 85mm minimum for portraits, and actually usually longer. A 24-70, to me, is a really boring general range lens. I'd rather own the Sigma 35/1.4 and a good 85 (I'll probably try a Sig 85 and if that fails, the 85/1.8) because I enjoy the DOF effects more than the convenience of not having to change a lens.
24-70, to me at least, is more of an event zoom where one absolutely must catch unpredictable action at various ranges, whereas there are better lenses for a more controlled, specialized use (like portraits).