Tbh I dont know how relief inf can win you game, considering this ability costs you 90muni mid game, where ost need it the most (schrecks\mines\mine sweepers and so on). This ability is somehow usefull only in defense, where you know that you would lose stuff no matter what and after 2 of your squads get half of the moddels killed you get ost squad to quickly move them to front line.
The only problem is that by that time USF would have few bars\LMGs, tommies might have brens (or 5 men or nothing, because they still would beat them) and soviets either PPSH or guards\shocks\penals which again would beat them.
Main advantages of osttupen commander is that you can spam them early, where you would both have on-pair army. While relief inf is awaible and possible at such point of the game, where osts are not worth it for such trade off.
Making them call-in unit in this commanders, wont change anything in general, because they still would arrive at such point of the game where un-supported osts (and even 2 of them) are useless. The only change it would bring to the game - osts would be more avaible.
The advantage of the doctrine is that, at least in old patch, it grants you up to 600mp worth of squads for the minimal cost of 200mp of reinforcements. Not counting enemy losses when they fight your soldiers that had to die. To avoid this many players try to avoid fights when the ability is active, but as it is quite lengthy, this ususally couses them to lose large portions of the map.
I do not know how well the ability functions in new patch but I will surely check it out.
Do mind that its all from 1v1 perspective. In team games I have never used it, but it is also due to the fact that lightning doctrine is not in the meta in team games.
Btw, IMHO relief infantry + pak is much better way of spending resources than shreck pgrens.