One of my club's small planes apparently has an intermittent problem with moisture in the static port line, so the recommendation is to always fly with the alternate static port open. That leads me to wonder why (1) the plane in question (a 172) was not simply designed to receive static pressure from inside the cabin, thereby avoiding this kind of problem and (2) if there's any downside (other than slight miscalibration of speeds and altitudes) to doing so.
I'm aware of the venturi-effect related pressure diff between an external static port and the cabin pressure. That just seems like a calibration issue for airspeed and altitude measurement.
tl;dr: Why are/were GA planes designed with external instead of cabin-side static ports, other than to give the examiners a tricky thing to ask us about?