I really dont think the minor is worth it, not to produce infantry at least, lets say you build inf on g2 and g3. then you paid 12 +9+9=30 for 6 infantry, might as well just build 4 mechs on g1, 2 on G2, they will reach east poland on the g3, and you will have saved 10 ipc for an ftr on gr (or 2 more mechs)
it has to be the major ic if you are planning to pump infs/mechs.
if you are not planning to assault moscow, and take the middle east, then you might chose to build a minor ic in rostov, to be able to produce 9 inf/mech/art each turn in ukraine/rostov/ stalingrad
I think the main difference between a major in rumania and building 10 inf in germany is that you will be able to stack ukraine on G4, instead of stacking west ukraine on G4, that means you can produce mechs/tanks in ukraine on G5, for the moscow hit, OR you might be able to collect stalingrad/cauc/middle east one turn before the schedual.
this ofcourse depends on if you are able to gen enough to ukraine on G4, you might need italy to take it, so that luftwaffe can land there, but then the benefit of having the ukrainian factory is not as big.
if the real prize is the middle east, then getting it a turn early will mean collecting all the objectives one more turn, that might be the difference.