The problem is, you’re asking about resale value. Most people who are willing to pay upper-5s/lower-6s for a dressage horse will probably want to see show records, unless they’re looking at very young horses.
Horses don’t have to show in order to ride at a certain level - everyone here knows that. But if you want to market them in the correlated price range for that experience level, you need to provide more proof to potential buyers, which is where scores come in.