In the case of table lamps, a polarized plug is usefull because (assuming everything is wired properly), the switch will be on the "HOT" side, which leads to the center contact.
The metal screwshell is then the neutral and this part will not accidentally become "live" if the plug is reversed.
This way, if the lamp is without a bulb, but still plugged in, an idiot will not get a shock from sticking his or her finger in the lampholder.
Of course for my own use, I think these things are an annoyance and I always curse whoever came up with these damned things. Sometimes I've replaced the plugs with standard ones.
However when I rewire and replace the lampholder on an old table lamp or attach a new cord on an appliance for a relative or friend, I ALWAYS connect the conductors to the proper screw terminals on the lamp socket and screw on a polarized dead-front (totally enclosed) plug, or use a cordset with a moulded-on polarized plug and make sure the switch is on the live side.
It protects me from anyone yelling at me because they got bitten by doing something stupid.
[This message has been edited by SvenNYC (edited 09-04-2003).]