We just did a warranty replacement timing belt on an '06 Aveo. Car came in with a driveabilty issue and my tech found a defective cam sensor. While he had the cover off he found an idler with less than 10,000 km on it with a bad bearing. Luckily, it was only growling badly and had not seized yet. Yes, it was a plastic idler. No, the plastic had not failed - the bearing did. The same bearing in a metal pulley would have had the same results. We had used a quality belt and w/p kit in the original install and the manufacturer warranted the part, but any manufacturer can have a bad bearing slip through. The point? The quality of the bearings not the material the pulley is made from is more important. A failed bearing will also destroy a plastic pulley making it look like the plastic, not the bearing failed.