Why should you change idler pulleys when doing a timing belt?

[elninoalex]

Because if you don't, they'll look like this. Notice the belt split in two lengthwise, but is still continous. The races separated on the lower left idler pulley and the outer part of the pulley actually came through the cam cover. It took out all the pulley and water pump bearings and jumped timing. But fortunately, it's non-interference so it's running like new after just $250. PO sold me the car like this for $1000 with 85k on it. Thanks dude.