Quoted from fordtudoor:
I have the metal ones, ala Stern, on a game and when I added the plastic ones (sinastra by Pincab Protectors) they cracked along the bend. So I wouldn't tell someone to arbitrarily add the plastic ones over the metal ones.
Not arbitrary, and has been discussed in several topics around here with numerous success stories (same way also that MustangPaul's leg light mod works). Yours is the first time I've heard of any issues having plastics over the metals, so I don't know how common that is. Not saying you are wrong or your experience is unique, just saying that I researched it thoroughly here on Pinside before I did it and many are doing it without issue.
Just to clarify, rogerdodger, if you have cabinet decals you must use the metal protectors or your decals will bubble - do not just use the plastics (FYI, metal goes on first up against the cabinet then plastic over that, then leg on top of that). Optionally, if you like the appearance, you may add the plastics on top (albeit, fordtudoor did have an issue). I already had the red plastics and liked the look, so I added them back into the mix after I applied the new decals and metal protectors on my Getaway.