The reason to replace the idler pulley, is basically because the pulley wheel itself is one great big giant sealed bearing. Think of a skateboard wheel bearing that is 5X big. Over time, the bearing inside can wear out, the grease itself can dry out from the heat, OR BETTER YET, if any of your front motor seals are leaking oil, all of the oil flying around WILL disolve all of the grease inside the bearing... Then eventually, the bearing will seize.... Not a good thing.
As other have mentioned, replace the idler pulley on EVERY timing belt change... It's a good safety/maintenance thing.... If not & the pulley fails, then you get to have premature failure on the timing belt. :shock:
