These promoters can relatively easily mutate or in some cases epigenetically change to reactivate the gene, so on evolutionary timescales it's a good idea to keep it around, for flexibility.

Evolution can't decide to keep something around just because it might be useful for future evolution. If it's not currently causing an organism to have more/stronger children (or fewer/weaker), evolution doesn't pay attention to it.

Also, you're describing pseudogenes. I don't think they make up a large part of noncoding DNA, but I don't have actual numbers.