b) it's not literally useless outside, you just have to exchange it for other coins to use it outside. Isn't that how cash in most countries work too? Besides, remember there's far less friction in exchanging app-specific tokens by another coin than there is in exchanging cash today.
Even if app-specific tokens do not become standard in the future, it's an interesting idea worth exploring. But the whole point is how different kinds of business models can be created due to the decentralized nature of blockchain, this is just one of them.
Yeah, you can trade your Cobinhood tokens for Binance coins, but this is an artifact of the fact that each built a closed ecosystem. If both platforms used the same currency (ETH, USD, whatever), then there'd be no need or the inconvenience to trade tokens around.
Also, note, not all dapp tokens have liquid secondary markets, so I'd argue that it's almost an exception that your dapp tokens can be monetized externally
b) "closed" means literally useless outside. Can you use your Cobinhood tokens at any other exchange? No, it's because Cobinhood tokens are only good for consuming the features of Cobinhood.