I used the stock idler for 3 years and this is what I have found to work best:
1: Don't treat it like a tensioner. It really isn't one. If you put your chain really tight, you will increase the chance of the idler bracket moving. Run 1/2" - 3/4" of slack.
2: the chainstays the idler bracket is mounted on are not parallel with chain angle. Most aren't. That means you need to bend the idler bracket to match your chain angle.
As long as your chain is straight and not too tight, you should not have a problem, IMO. I never had one. Bending the idler bracket just right is key.