I wouldn't call it useless it's more like it situational.
T34-76 is cheap. It cost less than Ostwinds, Command p4 and even Centaur. And it can already fight these thing. The fact it gets situational utility isn't necessarily unfair to Soviets.
T34-85 is simply the best medium in the game. It's got HP, Armor(less than p4 but still), Pen, AI performance, Mobility with cost efficiency to boot. + all the commanders this tank is available on are good. I can't see giving them a good vet 1 ability(I'm guessing combat orientated one that you think are usefull) be anything other than making it Borderline OP.
As you said, you can mostly use it with a doctrinal repair ability. If you use engineers to repair in a capping point, you don't need secure mode. Technically you could use it to optimize and maybe cap a couple of seconds earlier, but that is usually not worth the micro.
I personally have not used it in a long time (which is also due to not enough skill in micro) and I think I can remember it being used in tourney casts, but not really more than once in all casts.
The situations in which it really gives you a benefit are very, very rare to the point that I'd really call the ability useless.
The problem is that both the 76 and the 85 are already good and great respectively. If you can come up with a replacement vet 1 ability that is useful but doesn't buff their combat capacity we're all ears, because we couldn't think of anything.
I assume you mean cost efficiency? I agree that especially the T34/85 is a very good tank, but that's why I think that the idea of secure mode is quite good. I'd personally go for a quicker decap rate (and normal cap rate) coupled with a delay to getting the gun back. This would at least allow for some cut off play with the risk of being found "pants down" if the opponent realizes.
Improve the ability to make it useful, then increase the risk to have a trade off. If balanced right, it should reward proper judgement of the game while punishing misjudgment.