There's a good reason why this is uncommon (I'd say non-existant):
Consider this:

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What happens when you press 2
? Do you get 2
? Or do you get 12
?
The obvious answer is that you get 2
, because the affordance of a button or a menu item is an individual action trigger.
Here's another example:

What do you see first? Two irises or two eyes? Do the irises belong to the face, or are they part of the eyes, which belongs to the face?
The bounds of the button create a clear visual separation between Save
and As
. So you are creating separation to begin with, only to ask people to join the separated elements.
What your proposal does is force people to think and understand the (not immediately obvious) link between As
and Save
.
Consider the following issues:
- People who use calculators a lot may conclude that in order to
Save as
they need first to click on Save
then on as
.
- Why clicking on
as
equals save as
but clicking on save
is just save
?
- If you add more buttons to the toolbar, you have to keep them in a distance from this segmented button, so people won't see the menu as
Save as close
.
The important point is that you are only gaining the removal of the word Save
from Save as
, but you introduce a possible point of confusion (non-conventional control) and you are taking away something that virtually everyone is accustomed to: a Save as...
button.
My point is that there's hardly any benefit but quite a few penalties in you suggestion.