Illegal downloading of an artist's work, be it an author or a musician, deprives that artist of the residuals connect to the sale of his/her production. The person who illegally downloads music, for example, deprives the musician of earnings, earnings that he or she must share with others on the recording, not to mention production costs, etc.
If enough people illegally download, the artist may go into terrible debt and not be able to make any further recordings. People who download illegally should ask themselves, "How would I like it if others could access my original composition for free?"
The argument above is accurate and concise. My guess is that most people give little consideration to the ethical component of illegal downloading. Unless one writes or records or creates some other kind of product which can be stolen (taken without payment), he does not fully understand the ramifications of such actions. Most are not in this situation, so they do not have any compunction about taking such intellectual property without remuneration.
The main benefit of illegal downloading (if you are talking about benefits for the people who do the downloading) is that you get to have things that you did not pay for. You get to listen to music or watch movies without having to spend your money. This is clearly a material benefit to you.
On the other hand, the major con is that you are breaking the law and doing something that is ethically questionable at the very least. If you download something instead of buying it, you are essentially stealing. Even if you download something that you would not have bought, you are still using that without permission.
So, the pro of this is that you get a tangible benefit, but you have to weigh that against the idea that you are doing something that is illegal and unethical.
Music copyright and illegal downloads is definitely one of the most talked about topics in today's society. Unfortunately, downloading music (or anything) illegally is something that cannot be completely prevented. With today's technology it is very easy to get music, video games, etc, from almost anywhere, for free, and not get caught. This is probably the biggest reason why people do it.
That being said, I believe that the biggest pro to illegally downloading things is of course, not having to pay for something and still getting it. This is especially true for brand-new video games, where most cost upwards form $60-$70.
The biggest con then is the fact that developers/ music artists do not get paid for the work that they did. This is where many debates and arguments start, because some people believe that "big hit" artists can survive with some lost revenue, however, many independent and smaller artists depend on sales to pay bills.
Hope this helps!