I can see why. There's only so many commands you can give on an Xbox controller without button combos, which ES wanted to stay away from.

: The HW engine supported building placements (on top of the current socket
: system). Not sure if the decision to release with fixed placements was due
: to scheduling or the feature being incomplete.

It's been brought up in the interviews, and my understanding is that free placement of buildings was tried in early builds of the game. It got to the point where you could rotate the buildings before placing them when ES decided it was too complicated.

: Another ability, with some of its assets were left in, was placing mine
: fields.

Yeah, I remember that from your (Awesome) thread about cut Halo Wars content.

: Indeed, it would be interesting to see the buildings in base sockets support
: garrisoning of infantry.

That could help, but...
I was thinking about CoH's multiplayer and campaign maps set in towns, which are littered with garrisonable buildings. You can have infantry fighting door to door, setting up ambushes and making streets nearly impassible. A big part of this is the diversity of infantry, which Halo Wars lacks.