A lot of health care policy is based on the idea that it is always cheaper for someone to be treated in a doctor's office and receive preventative care, rather than developing into an emergency situation.
But yes, the people who were uninsured prior to obamacare are mostly the sickest people -- those who could not get insurance because they had pre-existing conditions, and getting sicker over time because they were unable to afford high-cost care out of pocket.