isn't the token that can be traded only a token that already has a price of $ and already has an exchange to trade it?
Myetherwallet gets the price of the tokens through API from a limited number of exchanges. i.e. Myetherwallet does not look for prices in every available exchanges but a selected few. If the token in context isn't listed on the exchanges supported by the Myetherwallet, it's price won't be displayed. Even if the price is not shown in Myetherwallet, you may be able to trade them somewhere.
Correct, there might be exchanges that you can trade your token even without being listed on MEW. But the issue is that MEW has became the gold standard for ERC-20 based token, and that is why the OP is quite surprised to see his tokens has no value in MEW. I hope the OP don't mind, what tokens are you holding though?