Oh, Lord, this discussion again. First of all, why is the burden of evidence over here? Why is the question "How do you justify music piracy", and not "How do you justify demonizing the sharing of music"?
Honestly, with some attempts at a music career behind me, this has been a question I've tried to think about from different angles, and they mostly come up on the pro-file sharing side. I'll recount the two I find most relevant.
1. From your (or my) perspective, I've yet to see a good explanation for why it's supposed to be illegal. Copyright law is a rather vague and complicated area, with a lot of shades of gray. Surprisingly, I hear it referred to as 'stealing' a lot. This is simply not true. Absolutely nothing is removed when something is copied. This invariably leads to the argument that potential profits are removed, which is a half truth at best, willfully ignoring that clearly, many download as a way to try before you buy. I honestly see little fault in that, and it's certainly not theft. We hit a common problem, though, the lack of reliable statistics and -facts- in the discussion. The few statistics the RIAA trump up are usually old, outdated and used rather dishonestly, when they're not made up from whole cloth. As far as I know, the other side really has no statistics to back up any claims. Either way, I have trouble seeing this as -clearly- wrong from the keyboard side of things. It's problematic at worst.
2. From the perspective of the musician, the idea that illegal file sharing is killing their livelihood is patently ridiculous, one forwarded only by those knowing little about the financial realities of the music business or rather dishonest people. The sale of music albums usually make up a rather small portion of a musician's total income. I'm a little too tired to do the estimates right now, but what most bands receive from album sales is usually a pittance, most profits being given to middlemen and the record company. In the other hand, income from merchandise sales and concert tickets - a much more reliable, stronger source of cash, at that - is something that an artist usually sees a much, much higher share of. For a musician, the income from album sales is often a minor part of his income. However, file sharing means a musician easily reaches a much larger audience, generating even greater income in merchandise and ticket sales. For a musician, the way I see it, file sharing is outright profitable.
So let's not pretend we're protecting the poor, starving musicians. They're better off in this world of YouTube and torrents. The ones in need of protection, and desperately so, are the record companies. When they rose to the prominence they have today, during the first half of the 20th century, the record company was a musicians only avenue to reaching a wider audience. No one else had the know-how and resources to put all the pieces (producer, studio, sound engineer, publishing, logistics...) together in one place. They were an essential part of the music industry, and the industry as we know it today could not exist were it not for the record companies.
A band like Arctic Monkeys demonstrate that you can perfectly well find a wide audience without the help of a record company. Up until 2005, before the release of Whatever You Say I Am, That's What I'm Not, the band relied on MySpace and distributing records personally to fans, and they did so with remarkable success. When they did sign onto a label, convenient for any artist, they signed onto a small independent label. I for one suspect that the mega-labels have outlived at least some of their usefulness. At this stage, I believe it is in their best interest (and the music industry's) to evolve, lest they slowly perish.
There's one question in all of this that dumbfounds me. See, there's no doubt that both the music, movies and video games industry have serious challenges ahead, in a world where all their products can be easily acquired and enjoyed free of charge. The question bugging me is, why is our first resort to outlaw what seems to me to be the common practice? File sharing is so widespread, criminalizing it seems like a disastrous societal step, as you're making criminals of a large percentage of the population. This does not seem conducive to a long term solution. What I would like to see is the industry demonstrating some forward-thinking, and try to find ways to work with it; the internet, after all, offers huge opportunities to easily meet larger markets than ever before. Let's take advantage of that, and try to work around the threats in a creative way.
TL;DR: I absolutely do not need to justify illegally downloading music (although I happen to not do it). There's little wrong with it, and criminalizing and demonizing the practice is unhealthy and unproductive.