Forks suffer from a problem known as transaction replaying - If you tried to spend coins on one fork, it would probably be spent on the other fork too, which makes it nearly impossible for exchanges to accept/trade both.

Exchanges would only trade in the original (core) fork until BU adds the necessary replay protections to their fork, and have said as much.  It isn't a matter of supporting one or the other so much as a necessity because of the technical problems.  BU, naturally, does not wish to add replay protections as it could reveal very low numbers of adoption and use, and also represents the point of no return.

Once BU adds replay protections, most exchanges will list both coins until one fork dies.  The price of BU coins are most likely going to be much, much lower (though the prices of both forks will drop significantly), which is going to put revenue-sensitive miners in a world of hurt.

It isn't going to be pretty for anyone.  Ethereum's price will go up.