Maybe this is just a me thing, but the idea of buying more dens is stressing me out. I'd ideally like to buy more of them for space, but I hate the idea of having all of them visible at all times, especially if I wouldn't be using most of them for long. It looks cluttered and busy, and I don't know about how other people feel about this but I'm less likely to click through someone's profile if I'm overwhelmed by the amount of dens they have. I don't like that I feel hesitant to upgrade something that should only be beneficial. It feels like once I get it, there's no going back, and I really don't like that feeling.
When you have open slots, they are invisible from your profile page. Why shouldn't dens be too?