You are right. ATM, the main reason more advanced versions of cheap tanks are used by the ATGM. Peronally, when I need ATGMs I use vetted MT-LB Shturms, or havocs.
I find that T-55AMVs are seriously undearmored for their price. They can be one-shot from front hits by most weapons if they get a critical, and if not two shots from any respectable tank will do the trick. Oh and after the patch Bastions aren't 10 accuracy no more.
T-62M-V, well, its not "seriously underarmored" but just "underarmored". Has a great ATGM, but again, I think its more interesting to get a dedicated ATGM platform instead. It has an OP main gun, 8/9/3 is too much for a 115mm round (oh come on its the same AP that best 125mm ammo), but its slower to fire, slower to move, and consumes more fuel.
Maybe it's a personal preference, but I prefeer spending a little more points on an already expensive units and ensure that they have enough armor to survive if I make a tactical error sometime.
I didn't say that T-64BVs can not be used offensively, but you have to be cautious, almost paranoid. They survive no errors in that role, as flank shots are common behind enemy lines. Just support them well, and recon, recon and recon. Ofensively speaking, I think T-80 series are more adequate, if u need a heavy into enemy territory.
I would like to see your replays. Im sure I'll be learning something useful from them!
I think t-64 series only fired the 9M112 kobra ATGM. Don't know if in RL they can fire the more advanced 9M119M Refleks ATGM like the modern T-80s (in fact Refleks is a modernised 9M119 Svir ATGM). But I didn't mind a little. Kobras are fine for me. As I said, if I want ATGMs I use dedicated units. But that's a personal preference.
[EDIT] Ukranian T-64BM BULATs use the "KOMBAT" ATGM produced there. Earlier T-64s used the 1A33 fire control system that only enabled to fire Kobras, like earlier T-80s. And read that T-80U ATGM was the REFLEKS, not the SVIR, as it was used by T-72B series.