From the www.howstuffworks.com
"1. If you were to take apart an outlet and look at the contact wipers that the prongs slide into, you would find that they have have bumps on them. These bumps fit into the holes so that the outlet can grip the plug’s prongs more firmly. This detenting prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between the plug and the outlet.
"2. Electrical devices can be "factory-sealed" or "locked-out" by the manufacturer or owner using a plastic tie or a small padlock through one or both of the plug prong holes. Construction projects or industrial safety requirements may require this type of sealing. For example, a manufacturer might apply a plastic band through the hole and attach it to a tag that says, "You must do blah blah blah before plugging in this device". The user cannot plug in the device without removing the tag, so the user is sure to see the tag.
"3. There also is a small raw materials (metal) savings to the manufacturer of the actual plug prong. Every little bit helps!
"It’s been reported that really old outlets used captive ball bearings and coil springs for the detent, but today it is done with a bump and springy copper contacts."
OK. Here's my take on these things:
As to #1: It depends on the outlet. Some just have two spring-action blades that grip the sides of the plug pin. So you don't need the holes.
#2: I've seen such locks in safety supplies catalogues. It's usually meant for items that are being repaired on-site. The lock's bolt goes through the holes in the plug and renders it "unpluggable."
For instance, you tag out the plug on the photocopier so that Secretary Maude doesn't accidentally say "hey who unplugged the Xerox?", proceeds to plug the thing in and electrocute Repairman Mitch.
As far as #3 goes...I have my reservations about that one. Most likely the holes have something to do with the manufacturing process. As JDevlin says, some tooling at the factory aligns the blades prior to molding the rubber or plastic around them.
As far as the little captive ball bearings cited in the excerpt above, I've found this to be true in some modern-day receptacles.
Last year I went to a supermarket here in Chinatown and bought an off-brand Chinese-import Nema 1-15R (notice I didn't say 5-15) receptacle strip.
Upon dissasembly of this chintzy piece of *#@&, I found a tiny spring-loaded ballbearing held in place against the plastic housing in each slot.
Now that we're on the topic of China, the "official" non-grounded plug configuration is the NEMA 1-15. China uses 220 volts/50 Hertz and Australian style
[/ \] plugs for grounded devices.
Two-prong plugs for the domestic Chinese market tend to have no holes in the blades. I've got some small "gray-market" appliances (like a beard-trimmer recharger and a small hot glue gun) at home with such plugs and they do tend to slip out of some wall sockets much more easily.
This is probably more than any of you wanted to know...