At risk of showing my lack of true knowledge....
Subframe mounts are under car and are major connections between suspension system and body. By all counts, these things get hard or crack over long period of time and impact handling and ride. I read once that if badly worn, they will cause car to "drift" as would bad alignment. I have also read several times that to keep car functioning properly, they need to be re-newed whether you think so or not (on older cars).
I don't believe water in car has anything to do with them as they are bathed regularly where they are located.
I'm going to replace mine (or have them replaced) soon. Project 333B.
'85 300CD @ 144k miles