Having pathing for 2 block high paths and 1 block high paths is also a complexity issue.
Its probably more robust to have the calculations for both mode run in separate code, instead of combining that into one "dynamic" pathsearch.
But separate code also causes the whole ai code to gain in complexity (making it harder to debug and maintain).
I could see the 1 block high paths come into the game later, if that does not break other designs, such as the "dungeon" POIs designed not to allow zombies to pass 1 meter high passages.
(BTW: allowing the player to crouch and crawl though 1 m high blocks, would make players build very unappealing passages in their bases. This can easily end up into a crouching simulator for players optimizing their defensive designs)