Steem Power acts as voting power. It can be seem as Freedom Points. Holding this produces an effect similar to compound interest where you get a sort of dividend just for holding it. If you have enough Steem Power you can live off these dividends in your retirement so Steem Power also acts as the retirement fund for bloggers.
Steem Dollars allow you to speculate on the price of Steem without ever having to dump Steem. In addition, if you use Steem Dollars you may not have to pay as much in taxes because Steem Dollars are not fiat but are only pegged to the price of fiat. This means people who are dumping are paying taxes which they possibly don't have to pay.
In addition, Steem Dollars provide 10% interest a year. If you cash out $200,000 from your day trading into Steem Dollars then you get 10% of that a year, you've got enough to retire off the interest on Steem Dollars. Steem Power or Steem Dollars are both attractive to the really smart investors who are in the know.
This is why the whales and people with the most knowledge in the Steem community are rushing to acquire as much Steem Power as they can. You don't see a lot of dumps from these people because they know something.
Nothing I say is investment or legal advice. They are the opinions (based on facts in the whitepaper) from a complete stranger on the Internet.
Anyone who would like to trade should do so on the internal market. You have zero fees, and possibly zero tax. You get 10% interest on just holding Steem Dollars. So why not?