The pool of master limited partnerships with an investment grade rating is pretty small. In fact, only 16 of them have been rated BBB- or higher by Standard and Poor's. That means there are plenty of MLPs like Northern Tier Energy (NYSE: NTI), Linn Energy (LINEQ), or Alliance Resource Partners (ARLP -1.59%) that are out of luck. In this video, Fool.com contributor Aimee Duffy looks at the MLP space and explains why the aforementioned MLPs may never see an investment grade rating and what that means for the investors who hold them.