What problems would too stiff boots create?
The most obvious thing is that the top could bite into your legs, or damage your Achilles tendon. But you might stop that by using Silipose gel sleeves or other padding at the top. For dance boots, deeply cut backs should help too.
When my boots didn't fit, stiff boots helped create a lot of pain. Blisters too. Because the boots only touched the top of my feet in two places (at the ankle bones), concentrating all the force there, and they rubbed. If they were less stiff, they would have molded to fit my feet better, as did the boots I used before them.
But I've met people who use very stiff boots without problems.
E.g., a coach who wears stiff hockey boots when teaching figure skating, at the rink I skated at during early summer. Stiff boots let him get away with loose lacing, or no lacing, and he says that makes it easier to stay on the ice all day without foot problems.
E.g., a rink guard who claimed people had a lot more problems with the rink's figure skates, which weren't very stiff, than with their hockey skates, that didn't. But he also said the rink still had a few of the old blue stiff plastic figure skates (which stopped being made 11 or 12 years ago), and he said people had fewer problems with those than with the new softer ones. Also that they lasted longer (the new ones are thrown away every few years at most) - not surprising, since one loosely laced skating session can destroy soft boots, by bending them too much.
And a long time ago, a figure skater I knew fairly well who did a lot of jumps and spins in loose fitting but very stiff boots. He too said good fit made stiff OK.
So it's a good question. Maybe too stiff boots don't need to be a problem? Especially if they are successfully heat molded to fit well, and some sort of top padding or gel is used up top.
Maybe it also depends on the skater.
Anyone?