We had an idler fail on a Buick 3800 application a couple of years ago. Absolutely quiet until the failure, and of course it ate the belt. Luckily, wife and I were only a couple of miles to the next town where we could get it changed.
Hardest part was trying to exit the motorway and make the turns to get to the garage with no power steering.
In this case, the idler was also part of an aluminum casting which served some other purpose (dont remember now what) and was not serviceable as a separate part.
I looked at your Haynes link, and even though it says it is acceptable, I wouldnt feel very comfortable going on the road with a belt that looked like that.