If would be really huge for Bitshares to get BTS on a big, dollar denominated site. Also euros. Right now you cannot actually buy BTS in any reasonable quantity except wiht CNY. If you want to buy with bitcoin, you have to send it to btc38, sell bitcoin, buy BTS.
Sure, you can trade a few BTC worth just fine via poloniex, ccedk, shapeshift, blocktrades, or metaexchange. But after a bit of buying you cannot anymore wihtout paying a big premium, because there isnt enough liquidity anywhere except btc38. (Yeah I didnt mention bter because they have withdrawal disabled and are too risky to trust funds to for any amount of time).