I'm sure we've all seen advertisements where there's people of different races together; it's a way of selling things to ethnically diverse audiences. Even though it's always nice to have ethnic diversity in advertising, I think it's a little old to just show an interracial group of people. Whites, Asians, Blacks, and Latinos being together is no longer what diversity is all about. Diversity today is also about people who belong to more than one race; and I don't mean an interracial group of people, I mean individuals who have multiple ethnicities in themselves (in other words, mixed-race people).
Whenever I see an ad that shows people of different races, I automatically think, "oh they're trying to show diversity." To me, showing different races together in an ad isn't really a good way of advertising anymore because it's kind of obvious they're trying to appeal to everyone; and it looks kind of preachy and over-moral if the ethnic diversity is too obvious.
Now, if we have mixed-race people (or at least, people who look mixed) in ads, the ethnic diversity is less obvious, because it's not clear what their heritage is. And it's more effective in advertising because people can look at a mixed-race person and have their own interpretations about what his or her ethnicity is; most people never think about mixed ethnicity when they think about race, so they would assume this girl is monoracial (yet have no idea what ethnicity she is), when she's actually mixed.
Another reason it's more effective to have mixed-looking people in ads is because it promotes diversity without looking like it's being sold. If we have an ad with White, Black, and Asian people together (without any mixed-race), it's like they're shoving diversity in your face and trying to look "modern." But if you have mixed-looking people, the diversity isn't as "loud" and it's more subtle, because different people will have different ideas about what ethnicities they are.
Of course, if you actually are trying to advertise to a specific ethnicity, then mixed-looking people might not be a good idea.