Only problem is, they'll still incur warnings/infractions. We don't really even give out that many for favorites threads; there are a lot more given for duplicate threads/low quality posts and this in particular would increase if newbies were constantly making favorites threads. We'd be getting a lot of "I like so-and-so because he's cool", more so than we already do.
But if we do change the rule, it should be changed for everyone. Anyone can make favorites thread providing they still fall within the rules.