You mean those necessary and valuable worlds which the Federation can strip mine the shit out of to fuel their empire? The whole Class system of planets to a degree, in part measures the ecological development of a planet, that any world in question could just be a few million years away from becoming class M... And then there's a question of pre-existing life, that bars the Federation from touching any planet if there is a chance that even in 40 billion years that some "here and now" microbes might evolve into Welshmen. Obviously there are worlds because of their distance from it's star, and other problems, which will NEVER become class M, which is why they got probably at least 26 classifications of planet types until you get to an absolute penultimate clusterfuck like a class Y planet from Voy: Demon. Have you ever wondered what a Class N planet is like? What happens to the soufflé if you leave it in the oven too long?