The old Tri-ang locos had wipers (0-6-0 etc) to both running wheels on the insulated side. If one of the wipers failed then there would be sparking on the two crank-pins and connecting rod bearing/rubbing points. That spelled trouble!
If the wiper on your loco is bearing part on the metal rim and part on the plastic then it is a very poor design. If I had that problem and wanted to fix it rather than demanding my money back I imagine I would place the wheelset in the lathe and sand the plastic until it was marginally below the level of the metal rim. Consideration would need to be given as to how secure the rim was on the wheel center as such sanding would be useless if the rim could move in relation to the plastic center.