I think that "the top journals" is very field-dependent. If you're in a science, Nature and Science are usually the top but Cell is limited to the biological sciences for obvious reasons. So if you're in, say, math, you probably won't get published in Nature but getting a paper in the Journal of the American Mathematical Society would be considered just as prestigious by mathematicians. To what degrees adcoms actually realize this - especially since they all presumably work in a biological discipline - I'm not sure.
I've also seen quite a few Nature and Science papers - especially in biological disciplines - that have no fewer than twenty authors. I doubt that the 15th author contributed significantly to the project and certainly did not contribute significantly to the hypothesis and experimental design. If an undergrad is the 15th author in such a publication, it's more likely that he/she ran a few Westerns or PCRs under direction of a grad student. He/she probably would be viewed as a better researcher/scientist if he/she had contributed significantly to experimental design and published second author in PNAS, for instance.