The issue is that old gas can make the engine run poorly. In the worst case varnish flakes can clog jets and make the engine run lean. In that case, the engine can seize.
While it is likely that mixing old gas with new in a car will work fine, I choose not to risk it and would like to dispose of the old gas. I am happy to pay for the service if there is a local gas station etc...that handles this.
Jetskier
