Those who are mentally ill grow older and older and physically sicker. You may find yourself on a geropsych unit and you will need all your medsurg skills in addition to your psych skills.
Adult psych patients take not only their psych meds but also the meds that people who have poor living skills (or just plain bad genetic luck) wind up with -- meds for hypertension, diabetes, congestive heart failure, emphysema, hep C, AIDS...so you have to be up on a wide range of meds and have to be able to assess for health problems in addition to psych problems.
Your psych patients may have colostomies, amputations, healing surgical or self-inflicted wounds (have had several gunshot victims, self-inflicted)...the list goes on.
Yes, your psych skills are most important for psych. But the other skills are as well.
And all those of you who are wondering if you need psych in med-surg...ask any med-surg nurse!