Brings up a point that there is such a thing as being too close to your dive buddy. Separation should ideally provide adequate room for your buddy to turn comfortably without kicking or otherwise making physical contact with you. Swimming too close can introduce a stressor that can reduce the comfort level of the diver, taking your mind off the dive. And if your buddy's constantly kicking you guess what ... it's your fault. Now you're interfering with their ability to maneuver, which can lead to other problems.
As a photographer, I sometimes have to ask my dive buddies to back off a bit ... I want room to maneuver if I should see something I want to take a picture of. Being too close can, in some ways, be nearly as bad as being too far away. Spacing, like most things diving, depends on a number of factors ... but unless visibility limits you to being that close, "arms length" would definitely fit within my definition of too close ... body length would be a more suitable separation.
... Bob (Grateful Diver)