What do you mean "not external wallets?" If they made this change, @poloniex and @bittrex (which are steem accounts just like yours and mine) would immediately enable Steem Power deposits and set up trading for them. I'd be able to send my Steem Power to poloniex and trade it just like I can currently trade my Steem or SBD. Steem Power would have a different price than regular Steem, but it would be traded.

You have to keep in mind that the blockchain can't tell the difference between a "regular" wallet and an exchange wallet.