I might get chewed on for this, but anything more than 18 gears is a waste and a compromise.
I believe that all you really need is three chainrings in the front and six sprockets in the back. Everything else is marketing gimmickery. Even with 18 gears, you will have a lot of duplication of ratios. The main thing is to have the smaller chainring to go up steeper hills (especially if touring with a heavy load). For many (not all - I didn't say all...) riders, I question the need for the large chainring except for trying to exceed 40 mph going down a hill.
In order to accomodate more than six sprockets (8,9,10) on the freewheel, sprockets were made thinner which causes them to wear and break easier.
Additionally, the chains had to be made thinner to accomodate the thinner sprockets.
Of course, you'll have a dandy time trying to find components for an 18 speed rig these days.