You generally don't need an idle bar plug for most engine set ups. I've heard of some people preferring them when mounting the engine inverted; I'm guessing the idle bar might help keep fuel from downing the plug. (?)

I have used them in a pinch, when it was all I had in the flight box. Didn't really care for them.

But to answer your question.... No, idle bar plugs are not better, just different.
Dennis-