I guess that his role got the status of recurring after roberts got other bigger commitments workwise and after the fans didn't shared much love with DLs character.
At least not as much as for the other regulars. :/
I mean see HRG and Sylar, they both turned to regulars mostly because of the fans.
On a other note, it was the writers and producers fault because they didn't gave DLs character much screening in the first place so.... how are the fans supposed to connect to a character if they don't even get to see him?

What bugs me in that case is that after the fans start sharing some love with the character that FINALLY got some depth, the writers/producers put exactly this character down. -.-
Oh well....