Human-shaped Nobodies aren't exactly easy to find, you know. Even a measly Dusk requires a strong heart in order to be created. Chances are the Organization picked up Demyx and Larxene because they were available. They didn't really have a lot of choices to choose from...
Besides, Xehanort has a tendency to get his plans done in the most convenient way, not necessarily the BEST way. He'd already tried to do a bunch of shortcuts before, after all. Even if they weren't the BEST options, Demyx and Larxene still could've worked, and they were already there, so...