I find exchange tokens to be extremely beneficial. In ProBit Exchange, I can pay my trading fees using PROB (their token). And I can lower my trading fees to 0.03% if I stake PROB. So, yeah it has benefitted me A LOT!!
Yes and it can only be done in Probit exchange, while in other exchanges it cannot be done that way, so regarding its usefulness it is only limited to Probit so that people can be interested in using exchange probit, even though there are many other exchanges that are much better , only Probit applies this for the sake of transaction fee competition with other exchanges.