Did you notice that the list of projects does actually include C::B projects and C::B workspaces?
I think the label "project" has not the meaning of a "C::B project" but a "development project" in general. If you think in these terms it makes sense to present C::B projects and C::B workspaces together, because a "development project" can be a variant of 1..N "C::B projects"... ;-)
Finally you would loose the space for a "development project" for the additional workspace label and make the File menu even bigger than it is already. Please note that these are only my thoughts.
With regards, Morten.
Edit: On second thoughts: How about renaming "Recent projects" to "Recent workspaces and projects" for better clarification?