I think it's a genuinely pathetic and shit metric, but it's hard to know what might replace it for simple reference. Perhaps some balance of trading volume vs supply but that still wouldn't be soundbitey enough for most people.
That's the problem. Market cap is a very oversimplified metric, but it's the best that is easy to calculate and understand. But with trade volume, we would have the same problem - a high value here can also indicate that a particular good/cryptocurrency is just used for speculation but nothing more.
A more complete evaluation of the potential of different cryptocurrencies is what the people at
Coingecko tried, but it has its flaws, too; because for example the different coins have different development strategies. So the Coingecko value can be distorted by some minor metrics. It's still a very good complement to market cap if you want to evaluate a particular coin.
An alternative approach I would like to see realized would be the
trading volume to goods and services instead of the trading volume to fiat currencies and cryptocurrencies. For example, one could try to aggregate the information of Bitpay-like payment processors (with CoinPayments in the case of altcoins), OpenBazaar-like decentralized markets and centralized e-commerce sites. That would indicate if a cryptocurrency is really used. And here, I think, Bitcoin would beat all competitors by a very high margin (I wouldn't be surprised if it's over 90 percent).