Rear Echelon troops are only good for manning the rear echelon? Colour me surprised!
No other unit in the game is as useless. You don't even need them for repair thanks to tank crews. You don't need them to construct buildings because Americans spawn with all their buildings.. All of their construction abilities are available of assault engineers or doctrinal riflemen.
RE squads can get minesweepers and shoot in fighting positions shooting rifle grenades randomly. Volley Fire made them a useful unit with pros and cons. Oh and I suppose they're okay for crewing weapons teams, but riflemen are better and literally almost as cheap to reinforce anyway.
Again, they're the shittiest unit in the game. Their cheap cost doesn't really make up for the fact that they are complete garbage with very limited utility.