Even if all festival-goers with dogs kept them on a leash and were able to control their dogs on the leash and keep them from jumping on passersby or fighting or biting, and even if those dog owners cleaned up ALL the poop that their dogs left behind, there's still the issue of urination and other excretions.

I would not want to walk around festival grounds where a dog had recently squatted and made water. At festivals around here, people sit right on the grass -- sometimes on blankets, sometimes not. I would not want to sit in a dog's piddle... or in the residue of dog poop that had not been properly picked up by the owner... or in dog diarrhea... or in dog vomit... or in a puddle of dog saliva that some breeds drool.

If I had children, I would not want my children to walk on, sit on, roll in the grass on, or otherwise make contact with any of the abovementioned things that dogs excrete.

Then there's the noise of barking which interrupts the enjoyment of the music that I would be attending for the purpose of enjoying.

Lots of disadvantages here, and NO advantages, for festival-goers who do not have a dog (and for those who are considerate enough to leave their dog at home). I say ban them from festivals. Let them roam free in a dog park, but do not turn a music festival for people into a dog park.