In a PM discussion I mentioned to Jon that I thought it was because those variables haven't been "cleaned" and so they could contain anything and wouldn't be safe to include directly in the output. I'm sure that part's true. But I thought they would be OK to use in a condition (for instance in the headinclude template when the 'do' variable isn't in $vbulletin->GPC yet). Otherwise you'd need a plugin to create another variable. But I wasn't sure so Jon wisely decided to ask if anyone else knows.
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