I get real frustrated when people say something like ITS ANNOYING BLAH BLAH BLAH. Of course this grass would be in small patches and NOT BLOCKING ANYTHING.
Also, somebody in a really old post said it didn´t make sense, and I have to say I totally disagree. It´s like saying there´s an invisible barrier preventing wild pokemon from entering. There are animals in all cities, even big ones, (pigeons, etc.) and even though grass is a bit awkard in some scenarios, like some genius above said, one could create different tiles.
Yeah not to mention the fact that grass never gets cut.
A Town, as someone (too lazy to check) said, is supposed to be a safe place.
So why would it be over-run with wild Pokemon? Also think of it from a more logical point of view (even though it's Pokemon); Pokemon are monsters, not animals. I mean it's okay when a monster pops out in a dangerous road where people are prepared, but come on, we're talking about the places where you go shopping!
So grass is a nono :I