Yeah, going for guild only runs is definitely shady, and you might get some unloyal/short-term players on your roster, which could be the real drawback against it. If you're wondering whether or not it's bannable, there's zero threat of it.
To be honest, I hadn't even considered this, and I don't think it goes on that much, but now that it's out here on this big public forum...you might see it more from now on, or at least attempts to.