Reported here: Bug 392965
"save a copy" (of the project file) to have a "inbetween status saved":
well... you work on a project... and you want to keep working on it...
but you know that it is good so far...
and you want to have a copy of it so that you could get back there if you need to...
"save as" means: "save as" + close + open the previous file...
MANY applications have "save a copy" function
so, in short: Why not? because it is NOT the same..