mid-stream is required to transport energe throughout the country, and they are mostly priced on volume (ie, how many gallon of gas flow through my pipes). Leading players like EPD and ET have long terms contracts with upstream players so they are protected from oil and gas price fluctation. This is why even though oil price is coming down recently, EPD and ET still hold up really well.
Of course, if we have an energy crisis such as 2016-2018, all bets are off.
Do avoid buying MLP stocks in 401K or IRA account, due to complex tax return issue
