Well the fact that you're listed at an exchange doesn't mean you shouldn't continue promotion. Bounty in other words mean promotion so i don't think the fact that they are listed should restrict them. Moreover, these "after listing" campaigns do produce some appreciable results.
You are right, actually the bigger challenge is right after the crowdsale because the expectation of the investor is high that the project will perform and develop and they will get a return of their investment.
Ever wonder why investors always like the project to get listed fast?
Because they want to get their ROI as fast as possible.