Sturms and Echelons are pretty much never built. Having 1 Sturm is essential. Having 2 is stupid in most cases. Having 1 RE is essential in 1v1. Not so much in teamgames. Having more than 2 is pretty much always stupid except for 2 doctrines.
You're equating frontline usefulness with being a mandatory tech progression(since vanila factions can't tech without them) and that is not very valid comparison.
Sturms carry early game well and are REALLY good at repairing, you don't need to have more then 1 for unit to be useful and you feel how useful and important the unit is when you lose it.
I do agree REs do not have as much importance, but they still make an excellent bazooka platform and that Pershing obviously isn't going to repair itself, mine sweeping is invaluable regardless of game mode.
The main question is: Is there any tactical/strategic advantage to being forced to build tech with pioneers? Is it a meaningful decision potentially having to retreat your pioneer just to build the next tech building earlier or simply wait for another minute until the unit needs to retreat from combat anyway? Especially considering that this decision needs to be made only 2-3 times in a whole match, and always within the first 15-20 minutes.
Yeah, it does boil down to this.
2 factions having to give up ground and retreat a squad off the field when they don't have to, while 3 factions don't has nothing to do with strategy or tactics, you're just forced to do so when you want to tech when you actually can.