Good question. I actually did read somewhere that it will be listed on another exchange. Unfortunately I cant remember on which exchange this was. I dont think that listing it on another exchange is contradicting itself as people are also looking at BNB as an speculative token and not only as a means of paying their trading fees on Binance. So trading on other exchanges might do good for the token overall. I will try to find out which exchange the talk was about.
I agree that its of course not necessary because Binance alone is huge already, but my point was that the more exposure BNB gets, the more its price will go up and therefore benefit us as BNB holders. More exchanges meaning more exposure, just my thoughts.