they don't know any better, but they do notice the battery life and laggy phones and may not go back to android. Just because they aren't asking for stock (out of ignorance) doesn't mean they wouldn't prefer it, and that it wouldn't help Android as a whole.
If they've invested in the ecosystem, they're more likely to come back. As these phones get faster and more efficient, regular consumers will notice this supposed lag caused by skins.
I feel as if regular consumers (who have no brand preference) look for a smartphone that has a form factor they like, can run the apps they want, and is at the right price for them.
If their previous phone had MOTOBLUR and they check out a new one that has a similar MOTOBLUR, they're more likely to get that one out of familiarity - and that's what Motorola wants. All of these manufactures want your next phone to also be from them and that's more difficult for manufacturers to achieve if every phone had stock Android.