If it's 1/10,000, then the additional risk from having the spider mutation truly is negligible. There are some folks that are simply disturbed by the ataxia of spiders, as it reminds them of ataxia in humans, which is usually associated with something unpleasant.
On the other hand, waltzing mice have their fans. Some people would never dream of breeding waltzing mice, while others love them.
It all comes around to the definition of a 'defect'. Is a change in body shape desirable, or a 'defect'? Obviously, we don't demand conformity of shape in our other pets--check the difference in shape between a pug dog and a greyhound! So, is the duckbilling that occurs in cinnamons an actual issue, or just a neat alteration of form that is desirable? Do bug eyes in a super lesser actually cause any harm?
Is a spider's ataxia substantially worse than what's seen in fainting goats, or waltzing mice?
I think calling some of these traits 'genetic flaws' is loaded language--it's making an assumption from the outset, which not everyone will agree with. Not all of these things are necessarily flaws.
They are genetic mutations. All morphs are the result of genetic mutations being propagated. If they are not obviously detrimental to these animals as PETS, why call them flaws?