OK. I guess my question would be in what laboratory are you expected to spend years (plural) doing meaningful research that doesn't lead to publication of any kind?
There are many reasons why someone with many years of experience may not have a publication to his/her name:
1) In many labs, being the one to physically collect data for a project does not automatically guarantee you a spot on a publication. That is up to the judgment of the PI and the postdoc or graduate student in charge of the project. If you are in a lab where a strong intellectual contribution is a requirement for getting your name onto a paper, you could easily burn through a year or so without a piblication record to show for it.
2) If the publication story is multi-faceted, there is a good shot that the work will not be complete and ready for publication before the application cycle rolls around. Thus, even huge contributions to a project may not result in a publication until well after the admissions decisions are made.
3) If the applicant is an undergraduate searching for an independent project...that project isn't often the lab's most promising scientific lead. PIs know that pre-meds (and pre-MD/PhDs) won't be around forever, and they would rather set them on a low-impact, but compact project, than one that is more suited to a graduate thesis.
All of those things combined with the fact that applicants don't always spent 3 years in the SAME lab, can make it pretty difficult for an applicant to guarantee a publication.
As for the OP's situation, you do not need to change the world to be a successful MD/PhD applicant, but you do need experience carrying-out and troubleshooting long-term projects. The point is to show that you still want a career in research even after you have been exposed to failed experiments, dead-ends, and research red tape (grant renewals, often-irrelevant research training, and publication hurdles come to mind). The point is that you aren't likely to experience those things fully through a lab course, and the admissions people need to be sure that you know what you are getting into (and will stick around) before they throw you into graduate school for several years.