Okay I'll just be blunt; needing 2 people to man MASSIVE structures such as the Garrison and Armored Fortress makes sense.
However, both the Blockhouse and Watchtower are smaller structures, with smaller defense radii, and are eventually outclassed by the Lightning Tower (which only requires 1 population anyhow and STILL beats them in range and space taken up on the map [2x2 as opposed to 3x3]).
To make both of these structures more feasible to keep around [looks wise/battle wise].... just require 1 population instead of 2.
That and it makes no sense that Pillbox -> Armored Pillbox is just 1 population, but the next step up, Blockhouses, requires 2x the manpower to man it.
At that point you're stuck deciding between 2 pillboxes/armored pillboxes or 1 blockhouse, when they should each be direct, CLEAR upgrades over each other.