The heims take the place of the bushings, so yes, they create a more solid mounting since there is no rubber in there any more. The cam bolt can be retained of you want, or it can be replaced. There is no additional stress on the part, but it does receive more direct stress compared to the rubber.
I beleive they do offer a threaded joint style with a bushing instead of a bearing. The biggest advantge to the double adjustables is you do not need to remove the arm to move the adjustment whereas a plain threaded bushing/heim, you would need the arm removed to thread the mounting point in or out beyond the normal range of the eccentrics.