I'm really curious because for me personally, if I were to delete something later then my opinion is that I shouldn't have posted it in the first place.
Twitter, for me, has the feel of an impromptu off-the-cuff conversation with some odd, semi-random, group of people.
It encourages the offhand remark, things that make sense and have value (hopefully) at a given time. When tweeted, a comment is valuable to the owner and the readers, but as time passes and the context is lost the value and meaning changes.
I like the idea of enforcing a public ephemerality on things that are created within a specific time and context.