Yes a single query will only take fractions of a second to run. But some people do have thousands of people online.
Granted we're not as big as
www.avsforum.com (yet) with only 600 or 700 people online at any one time and about 7000 home page hits per day, but it's still good practise to make the code as efficient as possible to ensure the server is not wasting it's time and your money.