You know what really grinds my gears?
Television actors who appear in commercials, clearly playing their current character, but under their own name rather than the character's.
Jane Krakowski is on 30 Rock playing Jenna Maroney, an exceedingly vain, psychotic, easily confused woman. Krakowski also appears in ads for orange juice as a vain, psychotic, confused woman named Jane Krakowski.
She claims to be endorsing Tropicana as herself, but behaves in the same way that her entirely fictitious character does. This means one of two things is true: either Krakowski's real personality is that of a punishingly narcissistic fool just like her character, or she is pretending to be someone else, and that someone likes Tropicana. Either way that's a terrible endorsement.
Put that aside for a minute. Why would you want your product endorsed by an unlikable character? Yes, Krakowski's Jenna Maroney is a funny sitcom character, but she's not likable. I'm glad she's in the show — as one-note as she is — but there are literally no people in the audience who want to be more like her. None with healthy psychological profiles, anyway. No one is sitting on their couch thinking "I want to be just like Jenna Maroney! I should start by drinking the same juice as her!"
Mindy Kaling does the same thing for frozen meals.
Oddly, Kaling's character on The Office is also vain, stupid and manipulative, like Maroney. Again, that's not a character I would want to associate with myself or my product.
I think half the cast of Modern Family has done the same thing in different commercials, including the gratingly precocious kid, the clueless dad and the Latin bombshell, so this isn't a trend limited to vain bimbos.
I am obviously no genius of marketing, but I can see no reason to do this besides hoping that the punters will make a lizard-brain connection between the product and some face I've seen before in a positive context and stop thinking before they realize they are getting an endorsement from a real person pretending to be a fake person pretending to be their real self, and that the fake person in an asshole. "Buy this because you recognize this person!" is half a step above "Look at this hot chick in a bikini! Buy our stuff!"