It matters cause I want my country to have the best stuff available at each category. Nothing separately is large enough to matter, but when things get combined, they work wonders.
Besides, the devs are yet to come with reasonable explanation to why they decided to make nations keep their initial unit type in the first place. This provides no flavor whatsoever and with pips now being put even closer together and some saying they are essentially the same you might as well make that final step and equalize them thoroughly; oh wait, that will just give us our earned western units back >.<
I do agree that given the units interact the exact same way, at this point the difference is somewhat academic. Especially for Eastern, however, the "worse units" is a pretty hefty phantom element. Even last patch with supposedly the worst tech 30 troops in Muslim it was really easy to wreck face in Europe even w/o NIs boosting military quality, and in MP making that kind of headway isn't realistic with the hugboxing + dominance of navies (which are identical to every nation).
At this point, it's more a design attempt (and a weak one at that) to give some of the native areas a small chance to fight back maybe. For example sub-Saharan pips are the same at tech 2 as western at tech 5. Of course, that becomes completely irrelevant when you introduce one tactics difference, but w/e.
Mid-late game battles will virtually always be decided by generals, terrain, tactics. The unit pip variance is indeed nearly irrelevant, so westernizing is for tech cost + trade companies only...probably not worth it for Russia.