It absolutely can be. Unlike older versions of SolidQuirks, you can sketch a stand alone datum curve, and use elements of that for any number of features (in the internal sketch) or even parts if done correctly.
There is this use in session tool in sketcher,but sketches are named in a way that you won't be able to distinguish them and if you ask for preview it crashes the whole application.
I seldom use external sketches because back in release 14 when I learned, they didn't exist. I reuse internal sketches often. Like Stephen said, if you save the sketch it becomes a .sec file, which you can import when starting a new sketch. The .sec file then becomes the internal sketch for the new feature.