I think that adding the calendar to the archive page wouldn't really be of much use - it only shows the distribution of posts for one month, and that information is much more readily obtained by simply glancing at the list of entries displayed in the archive page - that's what the archive page is for.
The calendar currently only appears on month-specific pages, so month, day, or post pages. Why? Well, frankly it's the only place it's of any use - it's providing information on posts made within the month that the user is currently browsing. If you're at a higher level than months (so in years, or just the archive) then you're looking at a whole different set of data.
And as for the calendar being applicable only for day-based blogs, well yes, that's true, and that's what this is - all the posts have dates associated with them, and so can be (and are) broken down into days. It's just another way of presenting info to people, and requires very little effort on my part.
I think, reflecting on my earlier reflection, that I may well keep the calendar for now. I'll try and implement some kind of JS-based system whereby I can record the clickthroughs from it, and next time space becomes an issue in that sidebar, I can review just how often the thing is used.
I think that might be a satisfactory solution.