If you are ripping firm groomers all day, yes, it makes sense to have a narrower ski, but if you are a skier looking for off-piste versatility, 65mm skis underfoot can be a little narrow and demanding. Say the day when it hasn't snowed in a couple of weeks, and you want to hit some firm groomers, some off-piste stuff like bumps, just skiing anywhere. Skiing springtime, when perhaps you want razor-sharp edge handling in the morning, followed by a bit of forgiveness later in the day, seems tailor-made to the 75mm ski. 65mm skis tend to dig quicker if you are skiing groomers that happen to be soft: 75-79mm skis don't do it as much, are more forgiving, but don't have the bulky feel of the 80mm+ skis. Try skiing a good 76mm ski on hardpack vs., say an 82mm: Cool Heat vs Cold Heat is a great example: skiing this on man-made snow, the 82mm is solid once engaged onto edge, but takes more effort to get there, reducing power and rebound in the process, and doesn't have the Porsche 911 feel of the 76mm. Yet, taking the 76mm Cool Heat up on a day with just a few inches of new snow in the past week, yet with soft groomers, and that ski will rip all day. The 82mm does the job, but isn't really wide enough to add much float, and isn't as quick, sporty, or as powerful. This isn't completely obvious until you get on a narrower power ski, which can be addicting once you feel the power of a race-like, high end ski pulling you into the turn, a feel that just isn't quite there with the wider skis. I would argue that many skis (specifically the wide carvers) from 80-87mm are just as much or more of a "lost width": a little stiff for bumps, not wide enough for deep snow, too wide to really feel powerful on the groomers. I think the 70-79mm skis are more focused skis toward versatile non new-snow performance. You could say the same about 90-100mm skis: not really good enough float for deep snow, not great in heavy windpack, don't really open up otherwise unskiiable terrain like a good wide (110mm+) ski.
If I had to have 2 skis, it would probably be a Dynastar 4x4, and something around 100mm underfoot (even up to Huge Trouble width). People who llve in many areas of the country just don't see much new snow, and therefore aren't going to need much width at all. Think Sun Valley: our Elan rep loves the 777 with Sun Valley as his home mountain: it is his everyday ski. He isn't a big guy, and it never snows there anyway, but that ski provides enough of a cushion for him to take advantage of those big 4 inch powder days they get, yet still get great edge hold on groomers and zipper the bumps.
I get your point about the narrower 65mm width skis, but due to the narrow tip, they can dig a bit if the snow isn't very firm, even on groomers. More width seems to equal a slightly bigger sweet spot than a 65mm ski, and people don't need that much power unless they are running gates.