Originally Posted by gromit
The port plate helps dirtribute the pulse pressure to the pair (pulsed air induction reed) valve which is directly above the plate. It is part of the exhaust emission system which reduces unburnt hydrocarbons into the environment. Removing it would make it less efficient BUT if you have an aftermarket exhaust (slip on or full) the entire system has been rendered totally useless because the catalytic converter was removed with the stock muffler. The catalytic converter must be present to reduce hydro carbons.
The PAIR valve still performs its function to inject air into the cylinder head exhaust port, regardless whether the bike has its stock exhaust or an aftermarket exhaust installed. I agree, the cat must be present to take advantage of the air being mixed into the exhaust to reduce emissions.
BTW, I checked my port plate when I did my second Valve Clearance Inspection... the port plate wasn't loose in the cylinder head cover. It is held in place by the reed valve assembly when the two screws are tightened down. Those three shiny spots on the port plate are just point of contact areas, which are not discolored by exhaust blow back, as is the case on the surrounding areas of the port plate. The reed valve itself could be a source of noise at certain rpm's, which in any case doesn't concern me. After 6000 miles, my bike runs better than ever.