As someone who actually requires real medical orthotics every day to help stave off knee replacement, and who knows a little anatomy, I'd have to argue that the truth about footbeds lies somewhere between most of the posters here, who think "orthotics" are the greatest thing since toast, and Lou, who says on his site that they do little except feel good.
What most here miss is that orthotics change the entire orientation of the lower half of the body, from foot to pelvis to middle back. The forces at every joint, including the vertebrae, are affected. You may not notice it walking around or skiing, but your joints do.
If you have comparatively normal feet, legs etc. a typical plastic footbed made by a well-meaning shop guy who read a pamphlet, watched another shop guy, and then started learning on customers, is probably going to feel great while a) doing nothing because it's too soft or b) being solid enough to slowly screw up your skeleton. As Lou says on his site, that nice high arch support is no favor to you. Or your skiing. And his arguments about heel lift are dead on, clinically. Ask any woman who wears high heels for a while about her lumbar region. (Well, OK, ask a woman you know pretty well.)
On the other hand, Lou's argument revolves around normal feet, attached to normal limbs and pelves and backs. Those subtle springboard and spreading effects of the arch, for instance, require an arch to begin with; people with overpronated ("flat") feet need not apply. Statistically speaking, 15% of the population will fall outside any one of those "normal" parameters, while 5-10% will be so far out to show signs and symptoms. And if you add up all the lower joints where we can be abnormal, the cummulative odds that any particular skier has one problem area that affects his/her foot become a lot higher.
So if you overpronate, for instance, it changes forces operating at the ankle, knee, and hip every time you flex in a turn. But this also works in reverse; an bigger femoral angle at the knee will create additional downforce on the medial malleolus (that big inside ankle bone that always kills) during flex, and cause too much pronation. This appears one reason why women, who have greater femoral angles because they have wider hips, tend to suffer more sports injuries to their knees and feet. More relative torque.
A real orthotic can correct some of that. The only mystery is whether you are truly part of that segment; I've never heard anyone think they had normal feet or were bad drivers.