Apparently, Goldman policy only permits so many top "outperform" ratings in any coverage universe. Therefore, when the Goldman analyst raised NBP to outperform, he was forced to eliminate one of the MLP stocks previously rated outperform. MMP was chosen for various cited reasons.
The Goldman analyst went on to list a number of positive reasons why he still likes (a lot) MMP.
Check out the dividend yield with NAT (Nordic American Tanker). They own and operate 3 Suezmax oil tankers. It's an arrangement so that they pay their income in the form of dividends. Upside: day rates for tankers fluctuate with the amount of oil being shipped and the limited amount of tankers to carry the oil. As the world demand for oil (China, U.S., etc.) increases, so does the demand on the limited amount of oil tankers; thereby, increasing the day-rate the tankers can charge, which translates into higher income/dividends. Downside: day-rates are volatile, and you have to figure the number of available boats (on the water, in the shipyards being built, and those retiring). If there are too many tankers to ship the demanded oil, then obviously day-rates would decline; thereby, reducing income/dividends.