If you get a freewheel with less gears, it is probably cheaper. The sprockets are probably thicker, and should last longer before they need replacing. So what is more important, having more gears, or spending less.
i'd have to refer to sheldon brown on the specs, but a 5 or 6 six speed sprocket is only marginally thicker than a 7.
over on mtbr.com, there's at least one thread...i've only read one, anyway....that talks about utilizing only a 1x6 (or less) set up on a (eg. hope) single speed freehub. the idea being zero offset to the wheel and having six choices of "singlespeed" gearing. the former reasoning was more prominent for justification, but apparently six gears is all some people want or need. i also seem to recall jeff jones having some part in the idea. that reading was quite a while back, though