Is it safe to transfer coins directly between exchanges? Say you want to transfer some coins from one exchange to another, and you want to do that directly without sending them to an indermediate address. Bittrex' Terms and Conditions, for example, state


  You may be required to verify that you control the External Virtual
  Currency Address that you use to load Virtual Currency to your Hosted
  Wallet.


I presume you are not able to verify that, if the external address is under the control of an exchange. But under what circumstances would it be even necessary to verify your withdrawal address? And what other risks are there, if you send them directly from the withdrawal address of exchange A to the deposit address of exchange B without going through another address which is only under your personal control?
I have used BITTREX, and I have transferred between exchanges (HitBTC, BITTREX, Binance). I am not aware of any risk, as this happens as a standard transaction between two wallets belonging to exchanges. I have even noticed, that the addresses change (maybe they use HD wallets). My use case was simple, that one exchange did not provide the altcoin I was looking for. So I was able to transfer amounts between exchanges, providing the target address. I did that with bitcoins, ethers and bcash. I haven't been asked by any of the exchanges, and can't remember having seen s.th. like this from BitTRex. 

I cannot say, under which circumstances this would be necessary, that is a question which might need to go to BitTRex. But I think it is a layer of protection for the users. Two many people sent their coins to wrong addresses, and then spammed the support teams with requests about their lost coins.