My question is what happens to Altcoins which you're holding and paired with BTC . . . if you want to sell later down the line what do you get in exchange? BTC Core or BTC Unlimited or both?
You'd get BTC - the alt/btc pairs will still be trading. As far as I can tell, neither Poloniex nor Bittrex plan to add BCC, let alone pairs with BCC.
Seems leaving Alts on Exhcnages is the same as leaving BTC since you're leaving it up to a third party to choose which pairing (or which coin) you receive when you come to sell . . .
Also your BTC is tied up in Alts so you won't receive free BCC.
Let's say BCC get's some support and acquires value then you will lose out, and if it surpasses BTC (highly unlikely) and Polo/Bittrex maintain their support of BTC you lose out more.
Just speculating what to do with my Altcoins on Exchanges.