The indices should have given a grace period rather than a blanket grandfathering of existing multi-class-structured companies. It arguably discourages further participation in the public markets if a massive amount of control has to be ceded, and it implicitly gives the grandfathered companies a massive advantage (in particular Facebook, wherein one founder has majority voting control despite not having majority ownership)
When the owners don't control the vote, it's difficult to pass up on short-term opportunities without facing pressure from activist investors and the like. And being able to stay the course is a massive advantage to those companies that can do so without facing corporate attacks. That's a concrete advantage for the long-term prospects of Facebook, assuming of course that the owners' interests are aligned with the long-term value of the company.
And obviously having innate demand in the form of index funds is a great advantage that boosts the market cap (as you see time and time again when stock prices pop upon entrance and drop upon exit of indices)
When the owners don't control the vote, you have situations like Facebook or Google. It may not be necessarily bad, but the idea of those who have control being the owners is an indication of why it can be bad.
