I am thinking about exchanging currency using middle rate. For example, I like to gamble with ETH, XRP but I want to hold my profit in BTC. So it will be fine if I can just convert it on the casino, I don't have to withdraw eth to external exchange, convert there to BTC and withdraw to my wallet.
I can understand your thoughts, but it will be quite complicated for Bitsler to exchange internally. They will have maintain realtime exchange rates and this is a different function. If they implement only internal exchange then Bitsler will have to hold most of altcoin or they will have to exchange from other exchange. And peer to peer exchange won't work fine on such as platform, who know how long will have to wait.
I believe peoples are playing with their favourite coins and obviously they will like to hold that coins. So I think this is not a big issue. For some obvious case likely you may use external exchange, or they might use API from some exchange.
Let's see what's is Bitsler decision, but your thought is reasonable as well.