While I do like the idea of "Duplicate," there are definitely some major problems it can impose for some of us.
For example, if you have a very large document, and need to duplicate it, this will require using more than double the memory of the original document, which hangs the app if you're already choked on memory, where before you could have just done "save as".
I just had a 400 MB .txt file (I'm a statistician — I have some big datasets) open in TextEdit to test this out, on the off-chance Apple had implemented some nifty only-track-delta-changes-until-the-duplicate-is-saved magic. Something I need to do a lot for reading in data is to replace the Windows line breaks with true Unix line breaks, which simply opening and re-saving a plain text file in TextEdit will generally accomplish for me. However, I like keeping the original around for redundancy and safety, so I usually "save as". So I tried doing a "Duplicate" on this 400 MB .txt file and get a beach ball as TextEdit climbs from 400 MB memory to 1.3 GB, as dramatic paging happens, before finally going back down to 900 MB a minute or so later.
So if you're dealing with very large files, where you could have used "Save as" before, rather than Duplicate, you better remember to copy-paste the file in Finder instead.