Pelican Parts Forums - Why do they use plastic for chain tensioners?

Everyone has seen this scary Audi V8 photo.
Why do they use plastic for chain tensioners?
It seems like this is not just Audi, but most car makers.
Are you simply unable to use metal on metal?
Are the plastic guides a sort of the sacrificial layer, albeit one that lasts 100k?

http://i.imgur.com/t5XUfav.jpg

https://www.reddit.com/r/pics/comments/3fm4t7/inside_an_audi_b6_42l_v8_engine/

Quote:

The previous Audi S4, which came out in 1999, used a timing belt rather than a timing chain. This annoyed the hell out of everyone, because the belt had to be changed every few years, and the only way you can do this job, or any job in a 1990s Audi, is you have to remove the front end, and the engine, and all the glass, and then you have to do a little jig on the service drive that involves a socket wrench and some lederhosen. For this, you may bill 31 hours.

So what Audi did on the V8-powered S4 model, which came out in 2004, was obvious: they ditched the stupid belt for a chain, like most other automakers. And since the chain was now designed to last the life of the car, Audi decided to stick it waaaay in the back of the engine, up against the firewall. Apparently the theory was that the chain would be so robust, and strong, and dependable, that it would never have to be serviced. You can probably guess what happened next.

Yes, that's right: it had to be serviced.

You see, as it turns out, the chain itself didn't have any problems with dependability. But Audi, being Audi, decided to use substandard materials for the chain tensioners, and the cam adjusters, which are two related – and highly important – components that keep the chain turning around and around and around every time you drive the car. The result is that these parts eventually fail, requiring the replacement of virtually everything you see in this amazing image of an Audi S4 engine worryingly sitting outside an Audi S4

Hereâ€™s Why The V8 Audi S4 Is An Awful Used Car