Well, considering that I have a MYO slot (In that case, Vendetta) from a closed species (That being the Lagmas), I have a mixed opinion on closed species in general. I do like the concept and idea of a closed species (Such as Pacapillars), but I'm not a fan of most designs of a CS, but that doesn't mean I hate them at all. However, most can be handled very poorly, and many CS caused too much drama in the community. The only known CS that I heard drama from is of the Wyngros group, since I once saw a journal on DeviantArt saying that they left the group due to drama, if I remembered correctly. I usually prefer Open Species over CS, because everyone is allowed to make their own without permission, and it causes less drama.
Edit: Gotta agree with Wanco-Alien on Open Species. They deserve more love.