Beat me to it, Rusty! Yes, I agree (with myself)! I like stiff boots and always have, at least to a point. There are many reasons, but in a nutshell, the boot is the "handle" by which we manipulate our ski, and through which we receive feedback from the ski. The stiffer and snugger the handle, the more more direct this vital two-way communication between skier and ski.
But stiff boots are less forgiving, of course. They require a skillful operator. They transmit mistakes to and from the skis completely and instantly. And it is possible for boots to be too stiff, causing them to transmit every vibration and jolt to the skier. This is not only painful, but it is slower too, and can cause the skis to chatter and lose their grip. It's similar to the suspension of a race car--very stiff and tight, but some compliance is necessary.
Of course, that wasn't your question. Do we need "ankle motion," and in particular, foward flex. As Rusty Guy says, my answer may differ from others, and I've taken pains to support it in the past (search the archives). My answer is yes. And no.
Yes, ankle motion is critical to good skiing. I feel like I ski with my ankles more than most skiers. Both fore and aft and laterally, minute and subtle movements of the ankles have very large effects as they translate up through the "kinetic chain" of the rest of the body. You don't need much, but ankles are the key to tipping movements and fore-aft balance and pressure control.
Many will tell you that you need to flex your ankles deeply in order to flex and extend while maintaining fore-aft balance. Indeed, the ankles, along with the knees, hips, spine, neck, and arms, (and heels, if they can lift off the floor) are all involved in fore-aft balance in almost everything we do. They would be in skiing too, IF the need to substantially lock our heels and ankles for support and control did not outweigh the need for flexibility here. But it does!
Using stiff, snug boots (i.e. skiing) involves a new, learned pattern of movement. Yes--learned, not innate. We must learn to flex and extend through the widest possible range, without involving our ankles or lifting our heels, and without affecting fore-aft balance. Normally when we bend low (flex), our heels rise and our ankles bend, which has the effect of moving our balance point forward. The knees bend back, moving our balance back. And the hips and spine bend forward. Extending is just the reverse. The combination of all these joints working together allows us to crouch low and stretch tall while keeping our balance.
You might assume, then, that ankle flex is essential on skis. But it isn't (again, within reason). With a few new movements, we can accomplish all the same things, AND take advantage of the high-performance "handle" of a stiff boot. I will say that the stiffer and snugger the boots, the more critical proper fore-aft alignment becomes. But properly set up (see "A" in the illustration), full range of flexion-extension is possible, even with the stiffest of boots.
So--make sure your boots are properly set up (shameless plug here: check the Academy Planning Forum for upcoming news about the Friday Alignment clinic preceding the Saturday-Sunday Eastern Tune-Up). And find a great instructor to help you discover the right movements (shameless plug here...ETU, ESA...link in my signature below...!).
Finally, here's an image I've posted before, that fully debunks the argument that you "must" have flexible ankles to ski moguls:
Note that all of the skier's joints EXCEPT the ankles are very active in this animation. He/she maintains perfect balance while absorbing the moguls, and never feels the dreaded "shin bang." Because they're NOT involved in the large movements of flexing and extending, this skier's ankles are free to make the subtle, refined movements needed for good skiing.
Others will probably disagree, but I've stated my case! Search the archives for more extensive coverage of this important topic--it's been much-discussed.
PS--I see that, while I've been typing, Weems has stated much the same case. (Hi Weems--how are you?) He is right, of course!
Now, if someone would just disagree, we could have a discussion!