Here is a list of the leading bitcoin exchanges in the world by volume (source:
http://btckan.com/price):
1. BTCChina - 75832.63 btc 24H volume.
2. Okcoin - 55016.86 btc 24H volume.
3. HuoBi - 42069.31 btc 24H volume.
4. BTC100 - 30284.05 btc 24H volume.
5. BTC-E - 29210.28 btc 24H volume.
As you can see, Okcoin which allows both bitcoin and litecoin trading is currently the number 2 bitcoin exchange in the world by volume, behind BTCChina which allows only bitcoin trading. Okcoin is already the leading litecoin exchange in the world by volume (source:
https://www.okcoin.com), having recently overtaken BTC-E. I predict within 1 month, Okcoin will also overtake BTCChina and become the leading bitcoin exchange in the world. Investors want to be able to trade BOTH major coins. Exchanges which allow the trading of only bitcoin will gradually be replaced by those that support both. This is simple natural selection at work.