I always say that you only hold coins if you have them in your own wallet that you have private key for but if you are lazy or not enough tech savy trusting an exchange during forks/airdrops may be a thing for you.

Currently this fork/split thing is getting out of hand, there is so many coming out it would be a real hassle and pain to get each right away so it might be a good idea to be able to trade fast aka let exchange hold a bit of your BTC.

I remember when BCH forked nobody could deposit or withdrawal coins due to the slow network yet people who had BTC on Bittrex got coins and could trade the markets right away. Risk vs reward as always.

Bitcoin Splits Source 1

Bitcoin Splits Source 2

Exchanges to Use

So far only two exchanges promised to honor every split, they do not say if they will allow trading but you will get the coins and when network is secure you will be able to use them. I have said about registration problem on exchanges but both of those need no ID and registrations are still open so get there if you want to get coins.