A lot of people have made good points here so I just want to speak in more general terms.
Rear Echelons are the most flexible combat engineer squad in the game but many of their specific roles require a lot of micro to work effectively. Plus, they're squishier than mainline squads due to inferior vet bonuses.
While Wehr pioneers can have about two combat roles (close-quarters, anti-building), REs have five (anti-tank, cover denial w/ grenades, fire support/suppression with BARs or M1919 LMG, support weapon denial w/ smoke grenades, and anti-building).
A few of them are micro-intensive (smoke, volley fire suppression, rifle grenade placement/bug of them walking forward to the squad you right-click on).
They're much easier to use, especially later in the game and as you add players, with just bazookas; your muni is better put into mainline squads with M1919s or BARs sooner into the game, too.
Usually, if I can put BARs on my RE squads, I've already won the game.
