There is nothing wrong with exchanges having their own coins. It can actually be good for business. But there are a few other factors that might suggest that,why doing so may not be in the best interest of the Cryptosphere. Here is a list of reasons as to why not every exchange needs a token of their own:
1. Frankly, not every exchange has the capacity to sustain its own coin in the long term. Anybody can launch a coin. The question is whether they can sustain the coin in the future or not.
2. There are already so many trading pairs avaialaible. You don't really need a liquid token of your own just to maintain smoother transactions.
3. Then there are so many coins out there in the market that exchanges don't really need their own separate coins to reward users. Anyone of the decent available coins can do that.
4. It is time to consolidate the number of tokens out there. 2017 was a crazy ride and so many coins popped up into the scene that it is difficult to keep up the count. It's time to support already existing good projects. Not to create more coins for purposes that can easily be solved by already existing coins.