Do you really want to try to race a barely-armored halftrack around to the back of the tank so your soldiers can plink at it with their pixy stix? Get the StuGs. They’re not that expensive, and if you handle their facing carefully and repair them when necessary, they’re pretty survivable. The “self-propelled gun” description is an excellent explanation of their proper use: a StuG is an AT gun, except that it moves faster and is a hard target. Keep dangerous infantry away from them and don’t let anything fuck them in the ass, and they will put solid shots into any situation. Not only do they kill vehicles, but they’re also a relatively early-game wrecking crew for buildings. Like regular AT guns, they’re even sometimes useful for putting distant infantry under fire.
I think Defensive is pretty neat, although bear in mind that I have been playing far less CoH lately than I would like, and I have not extensively used the Defensive doctrines. The super-bunkers encourage you to establish strongpoints and defensive lines. I like to have a “no man’s land” of claimed points in range of my defenses if possible. These points exist as lures for my opponent. They go in there, and all of my defenses can hit them. I may drop a Registered Artillery strike on them if necessary – it’s cheap, fast, and powerful, possibly the best artillery strike in the game (because there’s almost no warning; the Allies hear the shells falling and all of a sudden they’ve lost three squads).
Depending on the game type, this may be all you need to do to win. If you can hold a line that lets you secure enough victory points, you’re home free. For other victory conditions, the line can still be useful by giving you a layer of defenses that you can fall back to for repair, reinforcement, and fire support.
The Defensive 88 is powerful, but the investment of resources is hard for me to justify given its vulnerability. Once your opponent knows where it is, he’ll hit it with any artillery strike at his disposal. This is true of defensive emplacements in general, but bunkers can shelter infantry from artillery, and the infantry can then come out and repair the bunkers. The 88 may be worth it against an Airborne player, since it can fight back against airstrikes. Advance Warning, Registered Artillery, and base MGs also make life difficult for Airbornes attempting paratrooper infiltration.
I always forget to use For the Fatherland, or lack the munitions, or wish to keep the munitions for something else. I don’t know how effective it is, or how much I’m missing out. When I remember to activate it, it’s usually in a situation where I have overwhelming firepower anyway and kick myself for wasting the resources later. (One squad of Rangers is trying to cap a point in front of three bunkers, half my infantry, and a StuG! AAAAAHH NEIN!)
I typically spend points as follows: For the Fatherland (1), Super Bunkers (2) (also gives base MGs), Advance Warning (1), Registered Artillery (2). This is, admittedly, pretty rigid, but I want super bunkers as fast as possible. Reinforcement at bunkers is the wind beneath my wings. I may put off Registered Artillery or even Advance Warning for the 88 in unusual situations (such as having a really great spot for one, or knowing the enemy has gone Airborne).
If you don’t make Rocket Artillery the last thing on the tree, be sure to have a good reason to go for it. If the enemy has Infantry, you’ll want Rocket Artillery soon to counter his Howitzers when necessary. (I think Infantry is the hardest tree for Defensive to fight. Lots of artillery vs. fixed positions.) If you can infiltrate a sniper to kill the Howitzer’s crew, that’s cool too, but Rocket Artillery will let you reach that crap in the back and touch it fairly hard. Rocket Artillery can also support an attack. If you have a lot of Munitions to get rid of, you can shell his base repeatedly, which is at worst a nuisance and at best a building-killer. It’s no V-2, though.
I should stress again that my thoughts are not authoritative and I should really be playing more CoH.
Edit: They nerfed StuGs? Shit.