Guinness and AMV publicised the slowness of the pour with “Good things come to those who wait”. The National Dairy Council alluded to the high calorific content of cream cakes with “Naughty, but Nice”. (Incidentally, that strapline was coined by Salman Rushdie while working at Ogilvy & Mather.)
Admitting weakness is a tangible demonstration of honesty and, therefore, makes other claims more believable. Further to that, the best straplines harness the trade-off effect. We know from bitter experience that we don’t get anything for free in life. By admitting a weakness, a brand credibly establishes a related positive attribute.
Guinness may take longer to pour but boy, it’s worth it. Avis might not have the most sales but it’s desperate to keep you happy.