There's an instance a while back when (iirc) someone's dealership figured out their forum username and mods based on license plates found in photos posted online. As mentioned above, it's pretty much problematic if you're moded.
I know some people definitely prefer the anonymity. Sure, anyone can see your plates when your car's outside, but it's not like these people know much about you. Now if you've got photos of your car online and have a strong online presence that can be easily traced to those photos, someone can gather more information about you.
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