In reply to adam 24:
Logistics... you then have to climb with skis on your back, and ski boots on your feet. Or after your climb you have to come back to where you geared up.
Not always insuperable problems, but very often awkward ones that make it more trouble than it's worth.
Also, for half the typical Scottish winter on e.g. the Ben, especially late on (like from now till May), the snow/ ice is fantastic on the route but it only starts at about CIC Hut level. So you'd be carrying ski kit + climbing kit up to the CIC, gearing up with your skis, skinning up to the start of the climb, climbing, returning to your skis, skiing down to the CIC, de-gearing, walking out. That's a lot of faff for five/ten minutes on the planks.