Personal, anectdotal results from admittedly very limited experience ... in about 15% of 'buddy' dives I've been effectively a solo diver.-snip-
Yeah, your buddy should always be right next to you.
-snip-
Buddy just swam off and never looked back (actually that time I was having issues, fortunately only equalizing on a sawtooth tour dive.)
Buddy separation in the murk several times, once surprisingly, a couple not. All to easy in the fall at Dutch where viz in the training area is terrible at baseline and goes to hell when it is packed like a Manhattan sidewalk at rush hour.
Got tired of chasing an othewise competant instabuddy flying high in the Coz current and chasing underwater squirrels ...
So now I carry a pony which gives me full redundancy of gas, valve, & 1st/hose/2nd. And additional time to sort out a problem for myself or others.