The only remaining module wasn't specifically built for the shuttle -- Nauka will be going up on a Proton-M.
The issue with extending the shuttle was that there were a number of modules that were specifically sized for the shuttle, and there were some concerns of how it would look to our foreign partners who had built sections like Kibo, Tranquility, Rassvet and Leonardo, if we suddenly took away their only method of getting those modules up there. There was a question of how that would affect NASA's current and future collaboration with other countries.
Not all modules were built to be flown in the Shuttle; although the Shuttle did do the majority of the lifting, there were a half dozen or so that Russians took up.
update : I should've qualified it as 'remaining module after the shuttle is retired', there'd only be one left ... Jeremy is correct in that there's a module scheduled to go up in the last shuttle launch later this month.
There isn't currently a need to launch more sections. The ISS is now complete. The last mission of the Space Shuttle installed the final piece. One of the requirements for retiring the Shuttle program was that they scheduled enough flights to complete the ISS. Not that it is done, the Shuttle program can be retired. All that will be going up to the ISS now are crew rotations and supplies which will go up on the Soyuz rockets.