Whether you are going towards MD or MD/PhD, a publication is very helpful. A lot of schools factor research into how they rate a candidate. That said, there are different tiers of "doing research". Writing that you did research and being about to fluently talk about it is one thing (usually means you understood it, but you didn't or couldn't do enough to publish). Having an authorship (not first author) is another, usually indicative that you did a substantial amount of work to be put on the authorship. Having a first author publication is obviously the best, indicating that you did the majority of the work on a paper.
I would guess that schools factor these categories separately, but I could be wrong. For MD/PhD, publications are an unspoken necessity, because most directors want to see a dedication to research, and that is usually shown off in the time and effort it takes to obtain and authorship (whether first, second, third, etc...)
