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.