The bling effect.... Plus if the OEM ones are anything like the ones on my CBR600 - the quality ain't great...
You also get finer adjustment on a set of chain adjusters, as the adjusting screw is a finer pitched thread.
For the reasons already said above. I have gilles chain adjusters fitted to my blade and i had my wheel alignment check by laser line when my suspension was set up and the rear was perfectly straight . Prior to having them, i always string lined my wheels for alignment and never went by the standard markings as they are normally out.
