There is absolutely no reason to use Trust Wallet with Bitcoin or other established altcoins that already have plenty of standalone light and full clients. But when you are working with tokens on centralized networks like the BSC or Tron, you don't have valid open-source and verifiable alternatives. The makers of Trust Wallet and similar apps know that, and there will always be a market for them with those coin holders.

Unstoppable Wallet is often recommended as an open-source alternative, but its open-source nature doesn't make it a better choice currently because of various issues that the wallet has. It lacks the support for too many altcoins and shitcoins, it syncs too slow, and requires downloading hundreds of MB of blockchain data for certain assets. It losses its appeal as a light client if you need 20 minutes to sync your balance and download 300MB to your phone for each coin.

Although that might be better for your safety and privacy, those aren't the features that are on top of the priority list for such users. If someone uses a blockchain created by Binance, you aren't going to win an argument with them preaching about the benefits of open-source.