Traction bars don't really do anything, directly, for traction. They minimize/eliminate axle spring wrap, which helps to prevent wheel hop/chatter, which will help keep the tread in contact with the surface. If you have this condition, you'll have to compare the cost to the return according to your situation. If you are using the vehicle for high power/performance competition (drags, pulls, etc.) they are almost necessary. If you drive frequently on very loose surfaces, such as sand, they will allow better traction at higher tire pressure (prevents bead breaking) without all the chatter.

It depends on why you think you need them, how much you want/have to spend, and how they will affect the actual conditions. If you aren't experiencing wheel hop or chatter, they'll look cool, but won't help anything. I've had them on previous vehicles, and they got in the way more often than they helped. (YMMV) Are they worth it? Only you can decide.