New pairs are easy to come by, especially if you have access to the Decentralised exchanges. I don’t think you need to join a new exchange for that.
One argument that I know for registering on a new exchange is that volume on DEX is usually low, so you can't reliably trade there without affecting the market too much. On the other hand, if your coin is listed on one or two exchanges with little to no volume then it is worth questioning whether the coin is worth trading or not. I wouldn't register on a new exchange simply because my shit con is listed there, especially if their reputation is unknown but for some people that's worth doing if it means converting their shitcoin for money.