I agree with Barry. I happen to be a non-trad student with 6 years' experience in the OR before I even decided to go back to school, and 10 years now. Will my clinical experience help me tremendously when I have to learn to work with patients as an MS3? Absolutely. But I'm in school with traditional pre-meds who have shadowed and volunteered, and they've seen the same slice of the system (perhaps less intimately) as I have. Clinical experience can only help, but it's not imperative to your success as an MD or DO, which is why it's not even an official requirement. It's suggested because it's important to get a glimpse of what it really means to work in health care. While I'll have a small advantage in some ways, since my clinical experience drove me to pursue medical school in the first place, I'm still a little envious of my classmates who never took a break in their studies because they were determined to pursue medicine all along. They'll be my bosses when I finish 🙂
That said, I think shadowing is perhaps the most meaningful clinical experience you can get as an undergrad. I know people will jump on me for stating this, but the whole EMT thing is trite. I'm not discounting it as legitimate, but in my particular pre-med obsessed school, half the people I know are EMTs because they think it's basically a rite of passage for med school. My fiance did it, because his degree was in criminal justice and he wanted to figure out (part-time and at nights) if he was really interested in medicine, and this was the easiest way for him to do it while working full-time. It served its purpose for him, but he (like everyone I know worked as an EMT) says that it only helped him solidify his goals. It's great to become comfortable with emergencies, but it gives you little perspective on what medicine is all about. Shadowing a physician should give you a glimpse into what the role really entails.
On a side note, I'm pretty sure ADCOMs don't care what kind of volunteer experience you have. Whether you volunteer in a busy ER, or you tutor or work in a soup kitchen, the most thing you garner from your experience (as far as ADCOMs are concerned) is your desire and ability to work with people.