There isnt much to argue. They decided to take a character who was supposed to be in the game from the start and remove her from the finished product to urge people to buy it new instead of used. The game you buy in the stores does not come with everything that was intended for use on the disc. Bioware doesn't try to hide this so why should it?
In the middle of production they decided to cut a character from the game because there was no way to release the game on schedule for March with said character in. Therefore the character was moved into DLC. The PC version was delayed for other reasons but there wasn't time in the schedule to re-QA and bugfix the entire game with Shale integrated. The console versions had their own issues and there was equally not time to do that. Keeping Shale as DLC meant that she went through a separate QA and cert process that happened in parallel with the QA and cert process of the main game which had to be finished much sooner then DLC did.
You can read more about it
here but the main point is: Shale was not moved to DLC to create an incentive for new game sales, that was just an added bonus.