I am poor, which I use as my primary excuse to pirate software, movies, books, etc. To those who are not financially struggling: Why do you pirate? What is your moral justification?
If we're talking just about scientific articles, the need for piracy is obvious; without institutional access to journals, proper research of a subject is ludicrously expensive. In writing a publication, I generally use about 50-100 citations, and yes, you actually have to read them. And for every one that you did use, there are one or two that you didn't. So, simply putting together a manuscript would cost on the order of $5,000-10,000. In actual practice, this is something like a month of searching and reading papers. To sustain that rate is obviously impractical.
This makes those without institutional access completely deprived of useful access to the world's research. It's a terrible, indefensible situation.