During 2011, the FIA announced that blowing exhaust gases through the diffuser would become illegal for 2012. They attempted to achieve the ban by limiting the placement of the exhausts on the car, specifically with regards to height. Their attempts were ineffective.
While there has been a significant loss of rear downforce due to the current regulations, the teams have found ways to use the exhaust gases to produce downforce anyway, whether it be through having interestingly shaped floors or brake ducts or other means. Currently, Mercedes are trialling a system (similar to that already in use by other teams) that makes use of the Coanda effect to divert exhaust gases towards the edges of the diffuser, all without contravening the regulations.
While the teams are certainly innovative and creative, the continued development of exhaust-related aerodynamics should never have been possible. It would have been easy for the regulations to properly prevent the engineers from using the exhaust to produce downforce. All they had to do was stipulate that the exhaust gases must exit the car through pipes that extend to at least the rearmost point of the car. A quick look at Formula One cars of decades gone by would have made that solution obvious.
Perhaps the rules will be revised for next season, but as they stand they do not achieve their stated aim – that of preventing exhaust gases from having an aerodynamic effect on the car.