Yeah, they were only brought in because Bebop and Rocksteady's creator felt insulted enough knowing his creations were turned into comic relief based morons on the Tv show. Last thing he wants is for millions of people to see them look dumb on the big screen across America.
Essentially though, the characters are kind of reversals for each other. Brain power definitely is a bit higher in Bebop and Rocksteady's area then Tokka and Rahzar, though that's not saying much

. However, Tokka and Rahzar are twice the size of Bebop/Rocksteady AND all 4 of the ninja turtles.
Plus, there a thousand times stronger then all the turles put together. Another funny fact I found out, Tokka/Rahzar's costumes were the most expensive parts of the entire movie. Also, every video game made AFTER Turtles 2 had Shredder turn into Super Shredder at the end.