It is not recommended that you have so many accounts, it seems that you are a new trader, so it is best for you to concentrate in a single platform so you can get used to it and you avoid making mistakes when you trade, once you get a lot more experience you can use several accounts in different exchanges and find the one that you like the most.
If he has a very good trading knowledge and experience which is always the first thing that is important for anyone who wants to trade anyway, using different platforms should not be a problem. Most of the time choice of coins can make one to use certain platforms or mixed platforms. It is a good thing there is API in which you can easily trade on any of these platforms without having to log into them using other softwares like Coinigy on desktop or tabtrader on mobile.
There are so many things to always consider when it comes to trading, and for high volume trader, fee is one important factor after security, so in that case, I guess Binance wins the day.
After bittrex, I am finding all the comforts with binance. Even I have cleared the KYC requirements of bittrex, I am slowly moving toward binance. Whenever same trading pair is available in both of the exchanges, I do choose binnace for various reasons including all the ones you just mentioned.