The idea of the SMTs is similar to that or the ERC20 tokens on Ethereum. Where by new Crypto is made on the existing blockchain.
In the case of SMTs they shall be backed by a Steemit account and the Steem Power on that account.

So they are not "imaginary tokens" as some may describe a lot of the Crypto out there, presumably the ones you are referring to as a basis for your point of view.

The SMTs, have a goal to be utilized elsewhere on the world wide web and increasing the value and utilization of our blockchain, which = a higher value of Steem, which in turn = higher value of the SMTs out there!

It is a Win/Win situation.

As for the existing Pocket Tokens, you are not alone with your questions, having used my Pocket Tokens many a time to date I have found that many Steemians have many questions about them. Which is normal with anything that is new.
The proposed SMT Pockets here once introduced (if introduced) shall be an independant Crypto (SMT) and only in the "genesis" portion of the entire process shall be in some way "friends", the part where in which we are looking into ways to capitalize on our current Pocket Tokens.

To answer your question on trading, YES you can trade your current Pocket Tokens on Bitshares.

Just a note on the point you made with Steem should be Steem, I too was thinking that at first, but as I learned more about the ERC20 tokens on Ethereum and then having read the SMT Whitepaper numerous times I have taken a very PROactive attitude towards the new SMT initiative.