Think about this started. Interviews used to be done by a professional camera(wo)man then production assistants were given camcorders and told to see what they could grab during the interview. (MTV was particularly guilty of getting "runners" to do this. Not only were the poor sods working for nothing, just happy to have something to do with music tv, they were expected to provide footage which could be intercut with professional material). Needless to say their footage was mostly crap so using the Ground Force theory of "make a feature out of it" it was made monochrome in post.
That way it somehow became "intentionally" unsharp, wobbly and badly composed, being in black & White it was "cool".

This has now become quite acceptable in a lot of situations and the use of monochrome has extended in all sorts of directions. What you need to ask is "does this add anything to the interview?" If not, or if you're just using it to spice up a boring interview... then you're on to a loser.