Not only is the added weight a problem, but most of us with 3D/Acro machines benefit from the drag on the downlines to help keep speeds more consistant throughout a figure, and to add more time to the down lines for more figures and better presentation. Not to mention that with oversize control surfaces and travel, and very high power to weight throttle management is already a concern for a dedicated 3D setup. At lower speeds the drag isn't that significant, the weight is, and the drag at high speeds is helpful.

In the more modern full scale aerobatic aircraft with props like the MT, Claw, or Whirlwind composites a big part of the design intention is not only light weight and high thrust, but also high drag when power is reduced (of course these are constant speed props so when power is at or near idle the blades flatten out). You can *really* feel the effects when in level flight if you pull the power back quick you get pulled forward in the straps like you just got on the brakes in a car. This adds another dimension to the pilot's overall aircraft control.

Now, if you could add them without ading weight, well, then why not!

That's my .02