I love this question and I think about it a lot. I'm LBGT and too many times I see celebs being praised for being an ally or having a flag.
Whereas I get the basic message of their acceptance of the diverse group, there are times where you can definitely tell it's to be liked by that group.
A great example would be the blaxploitaion films (Celopatra Jones, Foxy Brown, Shaft etc). They offered diversity which was great because, like you said, many black people could finally have representation. But, the films were typically made half-assed and with a low budget. They often over sexualizes and exoticized the women. And to top it off many of them were directed and produced by white men to make money off the black community.
Same goes for a lot of current day LGBT representation. Yeah the TV show might have a lesbian character but all that character has to offer is their sexy and will probably die or have some sort of tragedy before finding or making love.
In short- diversity is praised because when the minority is finally represented it makes that community feel not as oppressed and they can relate to what they're seeing for once. However, there is a difference in exploiting communities for views and actually having a diverse media. Having a mix of dancers in a music video is nice so long as its based on their skill and not the artist's greed.