Primary care doctors are in a difficult spot these days. Paid less than specialists, many jam their schedules with 15-minute appointments to bring in cash. Hospitals are buying up clinics, and some independent physicians are eager to join and leave the business-side headaches to someone else. Independence Blue Cross, which insures 2.2 million people in the Philadelphia region, is betting that these doctors will respond to a deal: more money in exchange for more help keeping patients out of the hospital. The insurance company today is unveiling a joint venture with DaVita HealthCare Partners, which employs doctors in five sunbelt states.