As for the garrison being used rarely.. I think it may have something to do with the mechanic being simply hard to used.
Also, I wouldn't mind infantry units being able to take a building route when moving - In fact, infantry moving in an open area is something rarely seen as it is usualy a tactical suicide, and allowing infantry to move from building to building when traveling to their destination and thus using cover would be IMO a fine addition to the gameplay. However they should move slower when doing so.
I am also wondering about the ability of infantry to stay hidden inside building which would allow for ambushes.