I very much prefer outside over inside zombies, though I also do like meeting them inside. A question I had ever since the so called "static spawner" was removed is: Why? That spawner would spawn zombies around buildings when you approach. It was just so much fun to get to a house and you had to fight your way inside. I've never seen a reason why it was replaced with sleepers, but probably it has performance reasons.
Interestingly, you could already have zombies inside buildings in A12, and probably every alpha after that. You can just place them inside prefabs with a prefab editor. A few tweaks to the AI make them pretty much sleeper-esque, they won't wander around and only become active once they see the player. So you could have both: A lot of zombies as general biome spawn, zombies spawning around buildings and zombies spawning inside buildings. Particularly A12 did that with essentially no performance issues whatsoever.
Upsets me greatly.