I think you can't buy stocks from regulated markets with bitcoins. With dollars you need to register on a trader or investing bank, how do you do it with bitcoins?
In some countries, you may form a trust or a company. In some other places, the broker may hide your identity, and all your investments will be done under his name. It's getting more and more difficult though, with the ever growing AML and KYC regulations.
Maybe you can manage to find a broker to accept your bitcoin to buy stocks. Maybe you manage to find a good deal and win on the stocks let's say 5% in a longer period of time. Maybe the broker is not charging you any fees for the transaction.
But how you can be sure that for that longer period of time the exchange rate of bitcoin won't change opposite you? If it only rises 5% only you won't win anything as soon as you buy bitcoins again for your money.
Maybe if you want to do daytrade you don't need to count with the BTC exchange rate, I don't know if it's so volatile in a day too