Protectors, provided they are WoolSafe approved (ask the retailer if it is approved!), will not cause any damage and will show an improvement in resistance to soiling and staining, especially on a light-coloured carpet. Spots are also easier to clean.
Please remember, though, that these treatments are never quitefull-proof: when you have a spillage you still need to mop it up as soon as possible. If you leave it a stain may still develop.