My understanding is that the purpose of the overflow holes is twofold--
To allow water to flow down you drain faster.
To prevent overflow if the sink is filling faster than it's draining.
If #1 was true, wouldn't you also see the overflow holes on kitchen sinks?
If #2 was true, wouldn't you also see the overflow holes on bathroom tubs?
So--why do I never see overflow holes on tubs or kitchen sinks?
Edit: To clarify, my familiarity is primarily with sinks/tubs in the United States.
My guess (and that's what it is) would be that overflow passages are known to be unsanitary. In an area intended for food preparation, the cultivation of mildew and bacteria would be a more serious concern, where it isn't so much of a concern in handwashing sinks and bathtubs.
This article seems to support my hunch. It also suggests simple economics, as U.S. codes don't require kitchen overflows.
Most sinks and tubs in North America do have an overflow device, it's simply cleverly hidden.
Bathroom sink overflows (which aren't always present -- ours lack them) are visible as North American bathroom sinks are almost universally single basin. However, North American kitchen sinks are often double basin -- and in a double basin sink, the divider doesn't extend up to the full height of the sink, so the two sinks use each other for an overflow. A rather clever design if you ask me, provided you aren't filling all the basins up that is.
As to the bathtub? There's usually an overflow hiding in the drain-stopper selector mechanism.
I suspect this is simply because kitchen sinks are rarely operated by two-year-olds, and hence do not need training wheels.
In the UK I've never seen a kitchen sink without an overflow. They're universal on bathtubs as well. While they may not get used much in common use, they do come into their own if you get distracted while running washing up water, and distractions are common in kitchens, especially if you're trying to clean as you go.
I don't know many people who shave in their kitchen sinks, I do know a lot of folks that fill their sinks with water when shaving. I also know a lot of folks that fill their tubs, which is why overflows are also typically found on tubs. Some folks also fill the sink when washing up, in an attempt to waste less water.
"But I fill my kitchen sink to do dishes", you might say. That might be true, but the kitchen sink has a much greater volume. So you're less likely to fill it to the point of overflowing. Also, I'm pretty sure kitchen sink overflows were common back before dishwashers (but I could be mistaken). And if you have a double basin sink, they're typically designed so that the basins can overflow into each other.
It all about profit. It cost more to make a sink with an overflow drain. I am glad to hear that they are on all sinks in Europe.
They are very much needed on all sinks. And, this nonsense about it being unsanitary to have one in kitchen sink is just not true.
