Flares add bulk below the canopy and reduce bulk within the canopy.
It's basically an even trade-off but the theory is that you reduce wear on the canopy at the line attachment points and get a better airfoil shape. You also have to watch the flares when pro-packing. Since you're thinking about a rig, why not worry instead about Mike's history with rigs. I don't think you'll find anyone in this newsgroup who has a single serious gripe about the Javelin which is the other rig Mike designed.