Just to start, I completely empathize with working long hours in the lab, trying to get stuff to work and having nothing to show for it. I worked in medical research for 4 years before I had my first poster. Now, in response to your comments:
...however, I was thinking of saying just the opposite; that you should work hard for those particulars, but that what really matters is how much you learn and what your PI can write in the letter...Rxnman, have you gone through this? Where do you stand in this whole process?
I'm already a med student, so I'm waiting/developing for the next stage in applications: residency. I made a number of posts awhile back about
filling out ECs for AMCAS, and they kinda flow with my view on getting pubs. In the grand scheme of things, yes, you want to learn from your experiences. You
should do research either to see if it's for you, or to learn how to do it better.
But think about it from an ADCOM point of view (or a potential employer, if that helps). They see an applicant has worked in a lab for 4 years. How do they know if the applicant contributed to the lab or, (as you suggest,) was a bottle-washer? Posters, and similar lines on the CV, are proof* that:
1) You did high-quality work
2) You contributed significant time to a project
3) You participated in the intellectual development of the work
and as a result, you probably
4) learned more about conducting research.
I had something like a 2 posters, 2 presentations, and 2 abstracts as a successful applicant. I got questions about them whenever I interviewed and they were one of my selling points. I'm not saying that you're sunk without them - on the contrary, folks
are better off studying for the MCAT or boosting the GPA. But you don't want to waste your time, either. Posters and abstracts are evidence that you can point to and say "I do good work."
*Yes, some applicants will just present their mentor's work, but these are the minority