If they don't list/add some extra coins,stable and with good volume they will be behind from all the other exchangers.
I am not saying that you (as an exchanger) have to have them all (coins,tokens etc) but you have to have more than three and expand.
Of course i totally agree with this, that's the main purpose after all...
Well not necessarily. There are plenty of listings that list every coin and alt that pops up. These often included the random tokens that don't have much volume. If you look at coinmarketcap there are over 1000 coins listed and that doesn't included the myriad of coins that are launched weekly. GDAX have a reputation to uphold so would be selective in their choosing hence only the top alts that have a high average daily trading volume.