You're entirely correct about electricians using 15A receptacles on 20A circuits (because they're cheaper, and because the code tolerates it), and that misuse of the 15A connectors for 20A loads is not uncommon (even though it's a code violation).
The problem with routinely running 20 amps through a 15 amp receptacle is heat. We've all noticed that a plug may get warm during heavy use; this heat comes from the tiny amount of resistance between the spring contacts in the receptacle and the blades of the plug. Power dissipation varies with current squared; when current is 1.3 times over the rating (20 amps is 5 amps over the 15 amp rating) then power dissipation (the heating) is 1.69 times over the rating. Excess heat, ie more than the wipers were designed to handle, will cause their spring to relax. This becomes a cycle with deteriorating contact from weakened springs causing increased heating, which further weakens the springs.. heat may increase until eventually something melts and/or ignites. The heat often won't harm the blades of the plug; only the invisible springy pieces inside the receptacle are damaged.
There's my public service announcement: please don't make a habit of running 20 amps through a 15 amp connector! (on the same soapbox: if you have sockets where the phone charger literally falls out because the fit is so loose, please replace that poor worn-out socket ASAP! That too is a heat and fire hazard.)