I think I got it.
when the lighter paddles are too much lighter than desired, going back to stock is not an option, and there is no better alternative, flybar weights come in to play. am I right?
further, like graeme stated, they are used to fine tune the flying characteristics. even if lighter sets will do pretty good job, one can fine tune the machine with weights to suite his liking.
thanks guys. does anyone else have different opinions?