Individual correspondence is important, and I don't think Twitter would be a good vehicle for the types of things that might be communicated in personal correspondence.
That's actually exactly why I like Twitter. Direct Messages are a perfect way of keeping in touch with friends over something that isn't email worthy--the equivalent of sending an SMS if we were in the same country.
I'm not a Twitter user and this is perhaps one of the reasons I don't use it. Individual correspondence is important, and I don't think Twitter would be a good vehicle for the types of things that might be communicated in personal correspondence.
That said, it does has its purpose--perhaps I'd rather someone tweet a funny video, link, etc, etc, etc, rather than spamming their entire address book....