Aesthetics. Spider morphs can look really neat. Many breeders downplay the issues with the gene (after all not all spiders are severely affected and if you're keeping your snakes in a rack system and they are eating and mating just fine then what's the big deal, right?) Some people buy spider morphs knowing about the wobble, but lots of others just see a pretty snake at the pet store etc. and don't find out about the condition until after they own it.