Why do you allow the possibility to withdraw to external wallets only ETH-based tokens and not other coins as well? At least, Bitcoin would be nice to withdraw to our own wallets...
I would think that the transfer fees for ether are far cheaper than that of bitcoin. You can take the ether that is already converted and sell it for bitcoin on the exchange. You could also transfer the ether to another exchange and transfer it to bitcoin.
The DAO (decentralized autonomous organization) can only send erc20 tokens, that's why you can evert all non erc20 tokens to eth. The DAO runs on a smart contract so it will cost a fee of 0.002 eth to send it to your wallet. The DAO is here because this will make the COSS token nog classified as a security which can lead to trouble in the future.