But I have, on serveral occasions, been designing anf
prototyping
 something and found at the last minute that I need, say, a 390 ohm
 resistor. I don't have one in the junk box. In the old days, either in
 London or Bristol, I'd hop on a bus and get 10 from Maplin. And be
 designing again within the hour. Now I have to send an order to RS or
 Farnell and make it large enough to meet the latter's minimum order
 charge or so that the former's postage charge doesn't exceed the value of
 the components. OK, there are always components I can use, but finding
 the money every few days gets a bit much! 
    Resistors are SO cheap that pays for having a parts' tray with one
hundred of all values comercially avaiable and being replenished from time
to time. This is something you'll use forever. And doesn't get old.
    Greetz
    Alexandre, from Brazil, and his thousands of resistors :oD