1) "It's not stealing, because you only take a copy". We're playing with words here. First, the point is not the common definition of the word, it's what a court deems "stealing". Second, even in everyday life, we say things like "he stole the exam answers" or "He stole the recipe" (when in fact he only read them).
2) "The losses are imaginary, because not everybody would have bought the stuff he pirated". Sure, the estimations of the losses are imaginary. All estimations are imaginary --by the very definition. You can argue about the accuracy of the estimations, but not of the need of an estimation (even if you put that estimated loss to zero).
Also, the fact that "not everybody would have bought the stuff he pirated" bypasses two things:
a) some people WOULD have bought the stuff, because they want it, if it wasn't available for them in pirated form. Those are actual money lost (I have downloaded for free lots of things that I would have bought if they weren't available because I just had to have them. Surely, other people have too. How many, is the topic of the aforementioned estimation).
b) those that would never in any case had paid for the item, do not represent "actual lost money". But they have violated the wish of the content creator/distributor regarding the (paid) use of his product. This can also be punishable by law, and fined, and in many legal systems, it is.
This "respect of the will" of the copyright owner, is like GPL etc works. If you want GPL respected, you also want copyright respected, even if the owner is not using a permissive license. Because the court doesn't upheld GPL for it's permissiveness, but because of it being a copyright license.
3) "The creator gets nothing anyway, it's all the big media". This doesn't matter. For one, people still pirate things that are sold by their creators directly. People have pirated even the Radiohead "pay what you want" album.
Second, if the creator sold the rights to some "big media" company, that's his choice (or mistake) to make. That doesn't mean you have the right to grab a copy of his work as you see fit.
Third, for a lot of the content the creator is the big media company. People pirate like crazy BS teen-pop idols and hollywood blockbuster movies, which represent like 1% creative talent and 99% big media bucks for effects, marketing and production.
4) "Their business model is broken, they should find something else to make money" - Actually their business model is not broken. They make and sell something people want. The only broken part is that people copy it for free and give it around illegally. If I get me some GPL code, and make a closed source black box extended program out of it without giving back my source, just because I can, would people say that the "GPL model is broken"?
5) "People only pirate because the prices are absurdly high / the buying process is inconvenient" - Well, companies can set their prices to whatever they want for their own stuff, and they can make the buying process as convoluted as they want. That's not an argument in favor or copyright infringement, as much as high prices for Ferraris is not an argument in favor of stealing them, and the difficulty of attaining a university degree is not an argument for printing a fake in our printer.
6) "They can make it up on concerts" - Directors and shrink-wrapped software programmers don't give many concerts.
And even for musicians, a lot of musicians don't like giving concerts. Not everyone is like your local bar band or Rolling Stones. Especially in modern kinds of music, from electronica to ambient, there are lots of people that don't like playing concerts that much or feel that that is not their artistic medium (even the Beatles stopped playing concerts after '65). They still deserve some money for the use of their work.
Even for artists that DO play concerts, a gig used to be mostly a loss leader for album sales. The only people making big money from concerts are mainly established artists with a managerial team, everything organized, big following and sold out venues and bands with a largish cult following (think Phish, Pearl Jam, etc). That doesn't bode well for artists that still used to make a decent living selling around 50.000-100.000 albums a year. Some artists also have actual integrity and don't want to be seen dead selling "merchandise". Not everything is "rock'n'roll".
7) "What's the proof of piracy's harm? Record company profits are at a high this year" - How is this contradictory? One can have record profits and still be ripped off. The sets of "people buying record company stuff" (profits) VS the number of "people that WOULD have bought record company stuff but they download it illegally" (actual lost money due to piracy) are not the same set.
Those are some arguments.
My personal opinion?
1) Software patents must go. Especially trivial. A patent period of 5 years of actual use of the patent in production seems ok for everything else.
2) Copyright must be restricted. 20-30 years at max.
3) Illegal downloading should remain illegal. Not everything should be available for free against the owner's wish. "Metallica" releases and "The managing secrets of Attila the Hun" books are not an undeniable right for everybody, there are just stuff for sale. There are lots of copyleft software, music and books, one can use.