Given that the US does both, the exporting will decrease the net balance of importing. Should something happen to put a pinch on imports, of course what the Dakota pipeline carries can be put to a higher or complete degree of domestic use.
This kvetching is from environmentalists.
Exactly. Trying to segregate a resource by where it comes from only makes use of that resource less efficient.
As long as oil is allowed to be exported, it should not matter what pipeline it comes from.