You know, some of us are software developers. We may not work in games development, but we do work on large projects. We know how development teams are broken down in to smaller groups with specialist roles and areas of responsibility. It works the same way with any decent sized development project.
You seldom pull people out of one group just to bolster another group; it's a short term pay off with a long term detriment to the project. Once they've finished with their current sub-project you can then use them to bolster a group or assign them to a new group.
We know for a fact that the new content that's been coming on-line has been in development since before the game was released, because BioWare have actually stated that in one of their pieces to the community. Finishing it and putting it live actually frees up developers.