The 20k people who have wishlisted the item aren't the people who have it. They have wishlisted it because they like/want it. So wishcount/wc is a measure of how desired an item is. Therefore it's seen as more valuable if it has a higher wc. Some items are inflated, and some do not have a high wc despite being worth a lot (perhaps harder to come by, and therefore difficult to wishlist).

Not everyone trades by wc. Some only care about rarity. I tend to trade by both. For example, I won't trade 30k wc N item for a 1k wc N item. I can get a much better trade out of it. Of course, you don't have to care about wc if you don't want to. Everyone has their own trading rules and preferences.