I have seen many exchanges have launched their own token like Binance with BNB, Idex with IDEX, Hotbit with HTB, Yobit with YO etc. where exchanges use their own tokens as another way to pay fees to support token. In addition to it, different strategies are used to create a hype like decrease in supply etc.
But if an exchange goes through a hard period of time and shuts downs then ultimately that token will come to its end because there is no community other than that exchange supporting the token simply because it has no usage outside the exchange.
According to me exchanges should promote the usage of their coins/tokens outside the exchange to build a community in support of it. If your community consists of huge individual entities who believe you then a great community can be formed. Otherwise lack of applications would lead these things to disaster.
I believe exchange tokens are very useful and they are mainly built to be used in that particular exchange, most times its been used for reducing trading fees in the exchange, although not all exchanges tokens have a strong community,but an exchange like binance has found a way to build a very strong community for bnb by releasing several airdrops for Bnb holders, I have witnessed it, and in order for anyone to participate in any ongoing IEO on binance, they need to have a particular amount of bnb, I don't see bnb token becoming worthless anytime soon.