I find myself applying medical knowlege on a regular basis. For my geriatric patients, I'm routinely finding UTIs and very much aware of the side-effects and interactions of medications. I routinely check heights and weights of my adolescents, monitor blood pressures in my adults.
I don't always prescribe medications for these conditions, but a lot of times I'll suggest CAM methods, many of which I've used myself. It's a grade motivator to get patients to exercise, which will improve their mood, as wel, in addition to recomending that they follow-up with their PCP. Hibiscus tea works great for blood pressure. I've caught more than a few patients with low energy on statins or with CHF and had them start taking CoQ10, and suddenly they're feeling better again. I routinely recommend fish oil for my patients as the research seems to be showing mood stabilizing benefits, aid with anxiety, and improvements in lipid panels and weight with it. I routinely catch abnormal thyroid function and also check for low testosterone. I find many PCPs will miss low testosterone in young vets who have had TBIs, but I find it rather routinely.
It's hard at times to practice as an outpatient provider because you don't want to step on someone else's toes. No, you won't be prescribing the medicines an internist prescribes because that's not your job, that's their field of expertise, and really, the experts should be prescribing those sorts of medications. That doesn't mean you ignore the medical illness.
As for my stethoscope, well, that's collecting dust somewhere. You simply won't have time to use it much except on doing consults.
If you're looking at going into psychiatry and keeping abreast of medical knowlege, go ahead and subscribe to the American Family Physician Journal. I find their journal to be the most digestable and relevant to clinical practice of any that I've seen.