The reason you want more gears is to make it easier to keep the engine in the powerband. Those with wider powerbands, like the M900, probably could get by easier with 5 gears, while the 600, 620, and 750 might be able to use 6.
When I first got my M900 back in 1997, 6th gear often felt too high for cruising at 50-60 mph, and 1st gear was a bit high pulling away from a stop. Switching from a 39 to a 41-tooth rear fixed both. I later went from 15/41 to 14/39 when the chain was replaced and I thought that gearing was perfect.
The Monster is now gone and I've been riding an ST2 since November. The gearing was a bit high pulling away from a stop and I finally decided that 6th gear was also too high. Yesterday I installed a 14-tooth front (now 14/42) and it really fixed the gearing for me. I'll have to ride a few hundred miles of twisties on the bike, but I'm hoping that it will fix the awkward gearing that the bike seemed to have everywhere.
What were we talking about? ???
Oh yeah, five-speed transmissions. You get more overall range of gearing with a six-speed, so that when you finally drop the gearing on the sprockets you have a low enough first gear for easy starts, high enough top gear for highway use, and not too big of a jump in between any of the gears.