After your opponent has thrown a grenade. If you use if before he has thrown a grenade you are only left with the option for instant retreat or lose 3-4 models, possibly the whole squad on retreat.
I don't think they need them at the moment. However, the maxim is probably getting nerfed soon, which will mean that the soviet core army will be substantially weaker. So I'm going to say "yes" but only after the maxim nerf gets through.
The only problem is _and has been for a long time_ that call-ins do not require tech.
Take any tank and look at purely its performance vs its cost. You will find that they are all pretty much balanced right.
The problem comes when you can skip tech in favor of getting more tanks via a doctrine. This problem is most obvious with the allies because they have a few doctrines with medium call-in tanks. The Wehrmacht also is quite good at it, not teching and going tigers is still reasonably valid, and of course the most powerful 1v1 doctrine, mechanised assault with their 6cp no-tech STUGE into 11CP tiger, does it better than any soviet doctrine could hope to do. OKW can't do it, but luckily for them they don't require much tech to get their medium tanks out.
On a side note, even if assault grens are underperforming, I think the last thing the mechanised assault doctrine needs is a buff. From a 1v1 perspective, it completely outclasses the other Wehrmacht doctrines.