But I've just never seen plastic used as a shim material in any application with fairly high stresses except where the part was specifically designed using special materials. The bar stem joint is pretty critical because it has lots of leverage against it and because a failure could be so dangerous. Any shim can deform over time which is why they aren't the best idea in general. But you've got to think that plastic is less tough and more prone to deform over time than even aluminum. As has been stated in other comments, a lot of people have gotten away with using the 25.4 - 26.0 combo with no shim at all. I would guess that the plastic has mostly migrated to areas with less stress as if you had no shim at all.
There are two issues with plastic. First is the strength. Plastic simply is not as strong as metal (ignoring a few exceptions). Plastic used as a shim will tend to extrude (picture the material thinning out at the clamping interface) when overloaded in compression. Even if your shim survived the torque from the stem, keep in mind that even road riding will offer significant additional loads on the shim (pulling and pushing on the bars with your hands at least).
The other issue is creep. Overtime when subjected to stress, plastic will relax. Where you once had tension/compression, you no longer will. You do not need to exceed the yield stress of the material for this to happen (which is a big difference between plastics and metals). Heat tends to make creep much worse. I'd be curious for plastic shim users to test their handlebars after leaving the bike out in the summer sun all day.