I know this is an old thread but I just thought I would add my 2cents as a regular user of buddy lines on low viz dives. My buddy is the same on 99% of my dives - my wife, and so we are well accustomed to each others diving habits but in certain circumstances I consider the buddy line definitely reduces work loading. During (external) wreck dives with low viz so bad that it is just too easy to lose the wreck never mind keep worrying where your partner is. Often these were coupled with surface current so the best way back to the boat was via the anchor line. Combine this with depth over 20m and the last thing you want is to have to pop up to the surface to find your buddy.
I saw the comment about an uncontrolled ascent pulling the other partner to the surface. My experience is exactly the contrary and I am sure many lives have been saved by buddy lines. If I feel I am being pulled up suddenly I dump air, quickly shorten the line and dump my partners BC. A situation averted. I speak from experience.
Finally, in a situation where we spent a long time drifting on the surface waiting for some passing boat to pick us up after the tidal flow did not behave according to the tide tables, the buddy line kept us together with minimum effort, without constant finning.
What I use for a buddy line is a 2m elastic rubber bungee cord 5mm diameter (about 1/4"), with a knotted loop at each end. The loops are just big enough to easily fit over a gloved hand which leaves about 1m50 (4-5ft) of free line. The elasticity makes it easy to take the loops off the wrist. Often when changing direction and or going around corners the process of changing wrists comes automatically and simple.
So I always carry one in my BC pocket and use it when we feel it is convenient.
