I guess I can see both sides of the issue b/c I have done training with dogs at dog parks. BUT, this is NOT the preferrable way of doing things. And in a large park, definitely not.
As a pet sitter, I had 2 (and the occasional 3) dogs I took to the park on a regular basis. One I had been taking for over a year. I worked with recall training there, I had to. I wasn't about to go chasing her around ever time it was time to go. After a month, she had perfect recall.
The second dog was worse, and treats came into the picture, as well as leaving the leash on if there were few dogs there (which, in the afternoon was usually the case). She greatly improved w/i a couple of weeks was allowed totally off leash, but still easily distracted.
Our park, of course is very small, maybe 1 1/2 acres, so recall problems aren't usually a huge issue there. Before I bring my own dogs to any park, I would of course want good recall on them especially if there were strange dogs or alot of dogs there. I think in most cases, I don't like treats to be involved, only b/c Keiko will do absolutely nothing except glue herself to whoever has treats for the whole time we are there. I was lucky that the ones I brought must not have given off any kind of odor. If I had attracted a group of dogs, I would have abandoned that idea quickly. It can be dangerous to bring any kind of food into a park, including treats. My dogs aren't aggressive food getters, they will just sit and stare at you until you give them something. But I've seen other dogs biting at people's jackets/pants, jumping on them, and beinga general nuisance. Not to mention if there was a food aggressive dog there it would cause huge problems.