It makes sense for Bittrex to credit account with BCC, they will certainly add BCC for trading immediately, even if it flops, it will be a big altcoin for a while, and they have plenty of small altcoins listed already.
Coinbase on the otherhand probably won't ever list BCC, so don't want to open BCC wallets, just to allow withdrawls. It makes them no money, but is work for them.
If BCC really trades at 17% of BTC, I will be amazed! It will be heavily traded at the start, really people who don't have their coins on exchanges like Bittrex won't be able to trade them, if say 15% of the miners started mining BCC (I expect less) then blocks will be generated once an hour, so moving coins from offline wallets to exchanges will take ages!
If you have any relevant amounts of BTC, you don't want to keep them on exchanges, let alone locked in on there, I don't care if it's Bittrex or some random chinese crap exchange, don't ever trust exchanges.
If you want to instadump BCC for some free BTC that's ok, but don't do it with all of your BTC if you have many.
Just transfer your coins from your wallet once BTC/BCC trading pairs are added then dump stupid BCC for real BTC and enjoy the free coins. They will probably pump it before the dump so waiting a bit is not that bad anyway.