But wouldn't have it been more effective if it'd asked me to change it when I logged in, rather than waiting until I edited my presentation?

Well, that could be more efficient and sensible from the user end.

I suppose it could also prevent peoples' work being lost. However, since there are oodles of ways to have your work lost, and we can't prevent them all, you might want to get into the habit of saving your postings on your computer before pasting them into a page, just to be sure. (Or maybe your trusty Back button got your work back for you anyways... I don't know if every browser does that.)