I agree, Kyle was severly underused and the show dropped the ball on many storyline opportunies regarding his non-Braxton family background and his time in fostercare (huge missed opportunity there considering the shows long history of fostercare characters).
I really thought that once Kyle was the sole remaining Braxton that Nic would finally get a proper chance to shine with some bigger storylines and a wider range of screentime with different characters but instead he got shelved and the storylines were given to unliked characters (Ash, Phoebe, etc.).