I used an engineering vehicle in one situation to clear a traversable path through a heavily wooded band across the map for the two hover lances of my vehicle company. The rest of the tracked lance (which the Engineering vehicle was assigned to) was able to pick their way carefully through via light woods hexes, so the more vulnerable Engineers didn't run into any rude surprises on the other end. Never take any kind of "exit other end of map" mission without the proper tools to reach that other end.
In several games, I've used FS, Hermes II, or Vulcan incendiary 'Mechs to introduce some extreme temperature variations in wooded parts of the map where enemy units were sheltering. Unfortunately, the smoke tended to temporarily make the cover even more effective, at least until a turn or two after the fires burned out. I find Flamers and Infernos especially useful in 3025 play where adding 2-6 extra heat to a 'Mech with 10 heat sinks tends to do bad things to them....like Archers, Crusaders, Riflemen, and other units that typically run hot already, and carry lots of nasty explosive ammo.
More choices in terms of engineering tools, mobile bridging units, and more flexible APC/IFV designs would be very welcome.