On the topic of the importance of decoupling capacitors, recently a teacher handed me five PCBs which each take a 08m2, which is connected to a piezo, two LEDs and a switch (and no decoupling capacitor) and they weren't working. One of them didn't work because the manufacturer failed to test that there was a short between the pads for one LED, three didn't work due to sloppy soldering and the last one was odd - it looked fine but when the switch was pressed the tune would stop as soon as the switch was released and there was no problem with the power connections to the PICAXE. The board worked properly after adding a decoupling capacitor.
So statistically failure to include a decoupling capacitor causes one in five circuits to not work properly. Of course, the 6V AAA battery packs that were powering these things did not influence the results in any way whatsoever.
