Look at it the other way: With the old system, if you pressed "Don't save" to that dialog by mistake, you lost everything. With the new system, it's basically impossible to lose anything (the only way is if you don't notice for long enough that the old version gets reaped).
If you know it exists. And how long until it gets reaped? (I honestly have no idea. probably long enough for it to not matter, though, I'll admit)
I'll try another way too. You had unsaved changes. You chose 'save as', which you know makes a new file. Why would you want to save the current changes to both files, ending up with two identical ones? Filename aside, they're the same file, but one can be reverted and the other can't. That's what copy/paste/duplicate is for.
edit: there are plenty of valid cases for this, but they're mostly specialty (saving stages of changes in e.g. design work). why would the average person under average use want to save-as and end up with two identical files?
