IMHO, as a skier who owns 7 pairs of skis and 7 pairs of boots (I get rid of old gear as quickly as I can), desire for one ski that does it all is very rational, even leaving costs of skis and matching boots aside. Of course, that is one ski for every major division of skiing the individual spends substantial time in; if one skis only 10-20 days per year
1. Having many skis means making decisions every ski day and throughout the day about which ski to use ... how many skis do you want to transport to the ski area? And I could not stand running back and forth between the car and lift every day.
2. Here (PNW), lift-served, one can only rarely depend on consistent snow conditions; usually there are a variety on the groomed, between the groomed, off-piste, and in the side country. A versatile ski produces optimal results. So far, my Mantra has been stellar for what I do.
3. In the backcountry, conditions vary with aspect, elevation, and time of day, time since the last storm, etc. I rarely dither about which ski to use: one for winter, another one for spring-summer-fall; another one for nordic ski patrol on steeper, semi-groomed forest roads, sometimes with heavy new snow, sometimes with rutted, icy trails; and yet another one for well-groomed trails. In actuality, I could easily get away with my winter bc ski for all bc and one ski for trails.
4. My wife wonders how I can ski at all when I have to adapt to the very different characteristics of the skis, bindings, and boots I own. She of course has just one for lift-served (alpine ski, bindings, and boots), one for bc (telemark ski, bindings, and boots), one for forest roads (waxless, 3-pin, leather and plastic boots), and one for groomed trails (leather boots)--no variety there
Now, I would like to complement my 191 Mantras with a 178 MX-88 and will probably do so when I can ski again (after recuperating from a leg injury), especially if I move closer to a lift-served ski area (10 minute drive vs 1.5-2 hour drive). Then of course, I would probably want something like the S7 for those powder days (of which I have had none lift-served this year--although those at a ski area 3+ hours away and all the free/healthy time in the world have had a wunder year).