Thanks, everyone. I really enjoyed the responses.
First, I want to ask a question. Steve, you talk about the Hvy Flak being a very effective tank killer even though the player could use it in an indirect fire role. The problem I see with your suggestion is that if you give a Hvy Flak unit (that is capable of indirect fire; as some are not due to training), it will by default provide "on-call" fire support. How do force it to only function in a direct fire role?
Thanks.
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Now, some of my observations.
(1) ATG units have been vastly improved since RDOA in their ability to stand their ground. In RDOA, they were paper tigers when facing real tigers or anything else for that matter.
(2) Although support units need to deploy before engaging the enemy, I find that to really use them effectively it's best if they get dug-in in good terrain.
(3) Usually, they should be used at terrain boundaries: tree lines and town/city edges. However, I have wondered about the logic of perhaps sighting them somewhat further back from the boundary as opposed to at it. Despite being set back, there is still some LOS across the open ground. I wonder if they doesn't in part reduce their visibility and profile to attack forces. This might be an interesting research exercise. Also, as Paul pointed out with his road block example, it prevents the heavy weapon unit from being overrun while they are still able engage targets at range for the infantry holding the ground in front of them. I definitely need to try that.
(4) I have often used ATG and light flak units to interdict roads in a defensive posture. I find it best to do this for roads that traverse open spaces. I, also, find it best to not use them in a blocking capacity, but rather perpindicular to the road. These units don't do well at close quarters especially if they are being supressed by bombardment. One way to partially handle that is not to put them in the direct path of enemy columns. Thus, they can be disruptive with less risk of being overrun. Of course, the AI will mount an assault against defensive positions which are impeding its progress. But it takes hours for such orders to be put into action.
(5) These units can be very supressed by arty. The best solution to this is have your own arty on-call to bombard anyone they have a good contact to. Most likely if you are using them as I suggest in #4, then you are trying to halt enemy road traffic. In which case, hammering an enemy column which is exposed and scrambling because it is under direct fire is a superb arty target and works right into your plan.
(6) Not all these units seem frail. I have found SP assault guns and TDs to be very robust and effective when dug-in against moving armor and other light vehicles.
(7) I mainly tend to command these units on the defense. On the attack, I leave them with the AI. Granted they don't seem to do much, but when you dettach units from a larger force, you are then faced with the problem of how do you get them to the spot where you may later need them while providing them with adequate security? If you leave them as part of the attack force until after success and organizing the defense, then they will travel safely as part of the larger force. Now, of course, you could first [M]ove to the FUP area, then dettach them, and then order the [A]ttack; but this introduces additional delays which I prefer to avoid.
(8) These units can be used on the attack to cover the flanks of an attack if they can get positioned and dug-in before the attack is launched. This is best done when the enemy is holding largely open terrain and during a successful, the enemy will be dislodged and falling back toward the flanks. Dug-in heavy weapons on their flanks only puts more hurt on them when they try retreat away.
2021 - Resigned in writing as a 20+ year Matrix Beta and never looked back ...
Well, not exactly trapped, I am here, but it seems a little slow on the forum at the moment. 
so I will wait for the patch.
That's how I use them. I haven't had HTTR long enough to speak directly to HTTR so much, still experimenting and learning. That being said as in other games and so far in HTTR I use them to support both defensive and offensive units. ON defense use thier range to star getting firepower on advancing enemy units, try to get thier suppressiong up and organization down before they hit my infantry. Hold some in reserve and try to get flanking fire, ambushing the flank from a line of woods onto an enemy in the open hopefully where you fields of fire will have them. On offense again using thier range to suppress defending enemy and if my troops get bogged down try to flank the defenders and fire like hell on 'em.
