Timing chain guide rails - WHY replace these?

[Tomguy]

I consider myself a logical person.
I know the plastic timing chain guide rails can snap. Before examining, touching, molesting, or otherwise disturbing my current rails, I ordered new rails figuring mine were probably about to snap.

BUT... WHY would I want to replace my metal-backed timing chain guide rails, which look pretty damn strong and reinforced (and don't show too much wear for 140K miles) with a 100% plastic rail?

Seriously - I don't see the logic in this! The new plastic guide rails are about 0.5cm longer, but I think the original ones are about 1mm thicker.

Can anyone give me a VALID reason to replace my original, metal guide rails with plastic ones that are likely to snap? I'm not really LOOKING for an excuse to put a 5.6 in my Benz quite so soon!