1. If moviestocks are correlated, like stocks in the real world are (it doesn't matter if they are less correlated, only that they are even slightly correlated) , then comparing day-to-day or week-to-week or month-to-month movement of the index (or total market cap) would give traders a clue how much of their return is due to the alpha of the portfolio (it's return above/below parity) and how much is due to the direction of the overall market.
2. Longer-term, it would tell one something about the overall changing economics of the movie-markets. Are movies, overall, continuing to make more and more money? If so, how much? If not, how much? This is particularly interesting at a time in history when the future of theaters is uncertain.
Sure, the constant listing and delisting would make for a choppy index, but a choppy index is better than no index and over the longer term one should be able to distinguish the signal from the noise to at least some degree. The Dow and S&P 500 also list new and delist old companies, albeit at a slower pace, but the changing composition of those indexes over time doesn't mean that you can't use them to compare one time period to another.