And this is especially odd since it is clear that the two are a pair. Rutherford and Tendy are both B-storyliners. Their relationship to Mariner (the obvious lead) is like a pair of Banana Guards. That leaves Mariner and Boimler as the lynch pin for the comedic back and forth-- except the show works hard to prove that they really aren't a pair at all.
But to the extent they are a "pair," isn't it just an artifact of the duty roster? Boimler would be stuck with whoever he gets ordered to man the booth (etc.) with. It happens to be Mariner.
You have had a job, right? Sometimes you're just stuck with people.