TD offers better BWA against armoured divisions. Rocket Artillery is rather useless but in 1943 is is superior in terms of hard attack and art bombardement if compared to Art. SpRctArt is superior in 1942 and 1944 and from 1944 it has a better tc efficiency than SpArt. You can construct a situation for any brigade.
That's pretty much my point. Why invest a lot of resources (yes, research is the most expensive part, but even the IC could arguably be put to better use) into a bunch of brigades that will be good for about a year or in very specific situations I can't even create myself?
That's the main problem with AT: The tanks decide where to attack, because they're faster. I may or may not have AT there at that moment, but I can't shuffle my units fast enough. A human just uses recon in force to find my AT, and goes somewhere else - and the AI sees everything anyway. It might work to protect one or two plains provinces when everything else is mountains/swamps. But otherwise I'd rather be stronger against the 90% INF than against the 10% ARM.
For everything else: If I don't use them on at least 30% of my forces (or if they are absolutely vital for my strategy, like HArm for breakthroughs) I won't tech them. Currently, that's AC, Art and SPArt for me. Oh, and sometimes I get a few RAIL just for the fun of reducing a level 10 fort to rubble in a few days.