The benefit of chronological blog navigation
However criticized, calendar-based blog navigation has a great property that insures it keeps getting used over and over: it requires no extra taxonomy work to present navigation. You do not have to organize categories or come up with tags or maintain a carefully organized site map.
But the criticism is correct too, because the default presentation of chronological blog navigation gives you useless information, unless you are dealing with a personal-life based blog when you really do want to look back to see what the author was doing on a given date. There are people that this applies to, but probably not very many.
(A calendar for posts usually gives you useless information because for most blogs, the date something is posted on tells you nothing about what it is about. Plus, many calendar widgets don't even tell you how many posts a day has. The whole thing reaches the heights of absurdity when a blog is active enough to have posts almost every day and the calendar just turns into a solid mass of links.)
Better examples of calendar-based navigation are things like previous and next entry links for individual entries (ideally giving the title of the next and previous entries, so you have as much information as possible), or a chronological listing of entries with their titles or other useful details about them.
I suspect that calendars themselves keep showing up in blog templates for a number of reasons beyond simple habit. First, a calendar (in a small font) is very compact; you can wedge one into even a small sidebar. Second, calendars are attractive feedback for the blogger, who gets to see more and more days turning bold or whatever as they update regularly. (It's also feedback if you don't update, as your calendar widget pointedly illustrates your lack of posts.)