I haven't had cause to walk a dog in suburbia for many years until recently, when I've taken my collie Dot with me to visit t'old folks. I've taken her to open space on the city edge, where many others walk their dogs. I've been surprised and rather bemused to find that hardly anyone seems to think it a good idea to let their dog play with another!

As you said, honeyend, it used to be the way of it that you'd meet up with the same folks and dogs at the same times each day in the local park, the dogs had their special friends and all used to have a great runaround together while their humans chatted and threw the occasional ball. Some dogs weren't so friendly, and there'd be some specific pairs of dogs who didn't like each other, but these could generally be worked around by managing timings and so on - which mostly happened without discussion or arrangement, it just happened!
It's clearly very different these days, and not to the good of the majority of dogs, I'd say.
So if ventures such as this one mooted by anitasam are a way of reinstating healthy pack play for dogs, and enabling dogs otherwise only walked on lead to have a proper run about, I'm all for it. (But don't want the poo on my land

).
As to the amount of poo... if my dogs are anything to go by, they save the majority of their poo for walks

, and as we pick up after our dogs here I can say that amounts to two big poo bags a day from a pair of small collies over three walks.