People say they want more zombies, but the game isn't what it was back in the day when we had hub cities. Zombies were stupid and you could kill a couple and POIs were so short to loot it took 30 seconds to get everything and get out. Worst case scenario was maybe one might here you and beat on the wall out side. Rarely would they come in after you, or find you in a POI, and they were so basic getting out was a no brainer.
Now a quest and reward is at stake, and half a dozen zombies wandering in (or way more with the good AI we have now) would be a death sentence. In my opinion, even if we could add lots of zombies to cities, it would make the game worse. There is no pacing with a city full of zombies. You have to waste all your ammo and time clearing a block area so that you can even attempt to start on the POI, and often you would not have the ammo, meds and time left in the day to do it. I know, because priort to A18's release the thicker zed outdoor population was annoying as hell, and thats way thinner than what it was in the past.
So what sounds cool, isn't always fun. It ruins your plans and becomes a nuisance instead of something cool. I think the random encounter system will be better than just a hundred guys mulling around outside. Once in a while you might get sandwiched in, but not every time, and it won't always be kill 50 guys so I can get started on this POI.
I'm not saying it couldn't be cool, but it would be more work than simply spawning them and letting the AI take place.