The purpose of this is mainly so those who are updating to a new version can do so without having to also keep downloading the same block data, which by definition is going to grow over time.
I'm not sure how it is for you, but when I upgrade Bitcoin I don't have to re-download any blocks. It just picks up right where it left off before the upgrade.
That is the point. If the blocks are included in the update it would also by definition include blocks you already have obtained via the network. This is why I'm suggesting that it ought to be a separate but strongly recommended download for new users instead of something combined in the normal distros.