Unless the one diver is dragging the other one along on somehting they are afraid to do then I dont see a problem.
My GF is comforted by holding hands as am I. We like not having to look for eachother or worry about maintaining proper distance between eachother. Also we are both comforted by the fact that if all of a sudden one of us was to get in an OOA situation there would be no signaling or swimming, just reach over and grab.
I know I have seen most 'buddies' 10 or more feet apart in my little dive experience and I dont care how many dives you have under your belt, Id be willing to bet a good chunk of my paycheck that even the most experienced diver if they encountered a OOA situation 15-20 feet from their buddy who isnt looking at them and swimming along would panick. I know I dont want to be faced with that situation.
Try this some time, in the pool take your reg out and breath out all your air. Set your reg in front of you and just stare at it, see how long before mild panick sets in and you are grabbing for that thing, takes me about 20 seconds.
A lot of folks assume they have 2 minutes of hold breath time. What if you are breathing out and then as you go to breath in there is no air? I know this is unlikly but just in case, I dont ever want to be further than an arms length from my buddy nor do I want to have to chase them down or signal to get their attention.
On a lighter note, nor do I want to spend my dive worrying if Im getting ahead, or if my buddy has stopped to look at somehting, or if she is checking her air...when Im holding her hand I know all this because I can feel when she checks her air, I know she is right there, I know if she is getting cold, wants to go down to check somehting out, wants to go up to avoid coral, wants to stop to look at something, wants me to notice something cool, etc... Its all told to me with 0 effort and 0 processing time through her hand.