The idea is to “unify” the different standards for symbols: A symbol could have US and international shapes (ANSI, IEEE, NEMA, IEC, DIN, etc.), and other users could later switch them themselves in the schematic, if so desired (I know: less interesting for the US). While De Morgan isn’t much used anymore, there are still the distinctive- and uniform-shaped symbols of ANSI gates (AND/OR/XOR and negated versions). Also, 45° rotated shapes for bridge/ring circuits. One power symbol could have different voltages. Opamps could have alternates with +/− inputs swapped (when used as comparators). Transistors with and without envelopes (case circles). FETs with and without body diodes… That’s quickly more than a single alternate symbol.
For ngspice, it has the same builtin circuit element (D in SPICE) for regular, Schottky and Zener diodes, but they have different symbol shapes and currently require multiple symbols with the same functionality.
Of course that’s mostly for discrete components (“Device” library) and less interesting for ICs, but most of what’s mentioned above has been done already.