Yet again ? How you will trade one altcoin which has not yet listed in your exchange ?
You will ignore trading that altcoin or you will go for signing up a new account with a new exchange where that altcoin trading is available ?

There are multiple factors which are influencing why we need to maintain accounts with multiple exchanges. Arbitrage cannot be the only one reason for same.

Other than above mentioned reason, traders are choosing exchanges based on fees structure and legality of exchanges with the respect to their country. Hence obviously they need to have multiple exchanges.