They made that for the kids, supposedly... and / or because they didn't have time to figure out how to make the combiner work with respect to the movie release date. If you're buying the excuse at least.
I have the non-combining Constructicons figured out (unconfirmed):
Voyager Demolisher (lower body) - used to be a crane truck named Hook and formed the upper body
Voyager MixMaster (upper body) - used to be a leg
Deluxe Rampage (leg) -used to be an arm called BoneCrusher*
Deluxe Long Haul (leg) - used to be the lower body
Deluxe Scavenger (arm) always was an arm
Deluxe Scrapper (arm) - used to be a leg
*Rampage was a Predacon that turned into a Tiger.
In the first movie, they used the name Devastator for the Combaticon tank named Brawl. All the toys were packed with the name Brawl. Writers Orci and Kurtzman say that there will be an explanation after-the-fact (it was a subtitle mistake) in the new movie.
BoneCrusher was the name used by the mine-clearing vehicle (that destroys the bus in the first movie) whom Optimus Prime kills by beheading during the freeway battle.