According to the help text from the FFXI config.

"Enabling this option increases the likelihood that certain issues that arise with specific graphics cards will be avoided."

So possibly a fix for the issues for Nvidia cards?? Could be neat. I checked it, but haven't really noticed much as of yet, but I never really had issues with driver crashes and so on, just the usual weird lag around conflux ect. If anyone had those issues try turning on weather/shadows and see if the crashes persist.