I get the sense that most Jem'Hadar are actually quite intelligent and educated, at least about physical science, biology, medicine, and engineering. Those skills would make them more lethal fighters. If I got torn up in battle and a Founder ordered a Jem'Hadar to treat me medically, I would feel confident that they'd do a good job. Unless I was a Vorta. Although in Rocks and Shoals, maybe their problem was lack of access to medical facilities more than lack of a Jem'Hadar medic.