Leaving the lawn to grow out for more than three weeks makes it pretty unusable, especially for our dogs, with giant weeds (with spikes that hurt the dogs' legs) and thistle bushes and whatnot, and when you do mow it takes 2-3 times as long and the mower tends to get clogged up a bunch, and you need to extra careful about not hitting a rabbit or other nest, which might have been built in your yard because the weeds got so deep.
When I do mow I do try to leave some patches of clover for bees, though, and let some thistle plants grow in a few clumps.
I do wish people wouldn't obsessively mow and poison their lawns with weed killer to keep their lawns only consisting of grass, though. Nothing wrong with dandelions and clover and the occasional thistle.
As for polluting, I use a plug-in electric mower, so it doesn't really pollute much. I do wish electric mowers were the norm.