Usually you can actually clean plastic tanks. I usually take the option of cleaning the entire tank if I have issues. With age plastic can start generating gunk, but a bigger problem is usually the petcock and fuel pickup. However, metal tanks rust and are the worst. I have been forced to use filters on old street bikes with metal tanks. I've also had to use a filter on an old plastic tank. It was that or get stuck on the side of the trail with a carb issue.