wrote:
Anybody have any ideas as to why the prongs of an electrical plug (US
plug) usually have holes cut on its tips?

This site offers 3 possible reasons:

http://www.straightdope.com/columns/...ectrical-plugs

I've also read that they are used by manufacturers to "lock out" the plug
prior to its initial use so that they can attach a note to say something
like "Make sure you do (insert task here) before plugging in this device."
However, I think that that's a use of them, not a reason for them.