Switches are pretty simple, really: they're just a couple bits of metal that make and break connections. Over time, however, things can get gunked up with dust and carbon, or something. These build-ups can cause shorts or, more likely, can prevent the circuit from completing.
Some switches can be disassembled and cleaned, such as those for power windows in older cars. Others are far more difficult or impossible to open without destroying.
Try testing the switch with an ohmmeter: it should read about 0 ohms when on, and infinite when off.
I'd look at the analog board, too (taking precautions around the high-voltage stuff, of course). Specifically, check for burn or melted stuff, and check the fuses. Also, verify that the machine is rated for your local line voltage. IIRC, the original Macs were set for specific voltages.