On second thought, what about dreads?
Theoryhammer suggests that they are a decent support unit, only lacking in speed which may look bad when coupled with bikes, but it provides cover fire and a counter charge/combat threat with a 12" bubble around it.
Depends on what you want to achieve with the Bikers.
Bikers are a perfect harassing unit due to their speed and the cover save granted after turbo-boosting.
You can park them in front of a Tactical or Devastator unit and you can laugh about their effort to take it down via shooting.
What is the role of cover fire for a Biker unit?
Well, the Dread could pop a tank and the unit crawling out could be shot by the Bikers.
In case of some casualties, the Bikers might think about charging the unit.
Hmm, it depends whether charging is a good idea. Bikers have not enough attacks.
Moreover, if they do not charge, the enemy has to deal with them in the next turn.