Steem Power is not a different currency than Steem. It's just a different way of keeping Steem. If you give me your Bitcoins and whenever you request it I give it back to you in 12 weeks then it's just like Steem power. However, coins needs exchanges to be available to users and gain value. If they didn't needed they could just continue their way after ICO and sell their coins in their own website but they just don't do it for a reason.