check out Fiendix's
Land efficiency 1.06 comment thread - FAQ posted on 5.29.2004
In standard 1.06:
Eng. bde benefit only the div not the stack/unit.
The two river crossing techs enable inf. to cross without any penalty regardless of whether they do or do not have an eng. bde
Motor, mech, and Pz with eng bde get zero penalty for river crossing while they get a -30 penalty even with all research done without an eng. bde
While eng bde benefit defense, they generally do not help as much as an arty bde would. Combat consists of two parts. One part compares attack to defense values. If attack is > defense, then a lot more damage is done as the defense is unable to block part of the attack. If defense is = or > attack, however, then a small percentage of the attack gets thru.
Most units in standard 1.06 can end up with a defense around 24 which with an eng. bde increases to around 32. Most attack values, however, are 24 or less. Only high end Pz div (advanced heavy 100) have attack values high enough to be greater than an normal inf. div. defense. Units with an arty bde will also be higher than 24 for soft attack if all the arty research is done.
So in a typical situation, a div without an eng bde will fight almost as well as a div with an eng. bde Only when faced by an attack value between 25 and 32 would an eng bde benefit most units which have def 24 without an eng bde.
The primary benefit then would be for your mobile units. Pz motor and mech get no river crossing penalty with an eng bde while without one the best that they get is a -30 penalty. Motor and mech div also get +2 speed from eng bde.
As noted by others, CORE uses a very different system for this. I used to build a lot of inf-eng before but I'm not going to do it anymore. They add too little for their increased cost.