I have the same question.

I don't see buying Twitter as being very useful to Apple. Just because Apple, Google and Facebook are 3 of the largest tech giants at the moment doesn't mean they directly compete.

What's more, if the assumption is that Apple isn't very good at social, then the only thing they achieve by acquiring Twitter is to suddenly make themselves a straight competitor in a field where they have little expertise.

I don't see it making sense, but maybe I'm just being shortsighted :)