The Spitfire's main spar was unconventional only in its fabrication details. Tapered spar caps were not unusual in wooden cantilever wings, and a tubular-element truss spar was used in the contemporary Spartan 7W personal aircraft (albeit with steel tubing vs aluminium).
The use of hollow tubing is particularly weight-efficient for compressive loads (e.g. the upper spar cap in positive-G loading).
Another efficient structure is the corrugated inner skin, riveted to the smooth outer skin, developed by Jack Northrop about 1930, and used in the DC-1/DC-2/DC-3 series. There is no heavy spar, only a series of several lightweight spar webs to maintain the airfoil shape.