There's usable gears and then there's gears that you use. Not necessarily the same thing.
Then there's gears that are duplicates that can still be useful, to avoid shifting back & forth on the big ring.
One can't really generalize this topic, because there are too many different combinations of rings and cassettes. Some choices are far more intelligent than others.

I saw a cheap Huffy kids bike, that was a 3x6.
24-36-42 with a 14-16-18-21-24-28 rear.
Do the ratio's on Sheldon's link.
Add a 12T cog and make it a 7 speed!
It's pretty much a lesson on what NOT to choose!

http://sheldonbrown.com/gears/