When you have edited a post it only says "posted, date, edited." But it doesn't say when it is edited. So this would be much better. posted, date, edited on date/month/year or something similar, so you can see when the post is edited.
Of course, should be included in next release, meanwhile a add-on would be nice.
I believe the edited time is saved in the database (as well as the user name of who edited the post) already by the core. Shouldn't be too hard to write an extension to display that information.
I will add it to the end of my todo list (which is so long right now someone else may beat me to it!)