The problem with this idea is that some professions are simply more popular than others. In turn, certain professions story content would makes lots of money, while others wouldnt. In turn, Bioware would "follow the money" and make lots of content for the popular professions, and only a little for the not so popular ones. So the end results of such an idea would be the most popular professions having lots of content packs, while the not so popular professions would only have a few.
I don't think this would be a huge problem - most people that stick with the game play through most of the classes, at least to see the story up to 50. It takes a long time, but this is an MMO.
BioWare has the statistics, but the influx of new players in guilds and questions about alts on fleet seem to confirm it as best as we can know.