Much has been said about why people read scans. It's not my intention to lengthen that topic, but to reflect on why people should or shouldn't read/make scanlations.
With the internet, now more than ever, people have freedom. The law doesn't matter anymore, people will get what they want. Maybe, after a long time under the yoke of the law, people may have kept individual ethical reflection superficial.
I wanna show you some ideas, and I hope I can be balanced and clear. Some of what I say is specific to scanlations, some regarding file sharing in general.
Scans may be against the law, but the law has been wrong many times. The concept of authorship is very recent in human history and can be disputed. To believe that knowledge and art cannot be private property is a genuine way of thinking.
On the other hand, everything we have, roads, social security and even the sales and distribuition of artistic goods, depends on systems.
The power of systems doesn't come from above, but it belongs to each person that's a part of it. With illegal distribuition, people take their support away from the system, weakening and partially destroying it. Those people can't justify themselves saying that the system doesn't cover their expectations because, being out of the system, they can't demand anything from it. You can't demand improvements in public transportation if you don't use public transportation.
Another very used excuse is that sharing actually advertises the artist's work, possibly increasing sales. Well, isolated cases don't prove much, and the real effects are unmeasurable. It sounds nice to divulge an artist's work ("oh, they only make real money with concerts, anyway"), but that denies them control over the distribuiton of their own work. An ethical end needs ethical means. Like I said, it's okay to be an anarchist, but don't hide behind false moralism.
It's not necessary to take any of these extreme positions, there are intermediate positions. You could believe that free distribuition is the best way to go. If you like a comic, for example, it becomes a part of YOUR culture, and it's only natural to want to share it. Still, you could believe that authors have some right over their work, and that it's up to them to be or not a part of a freer world. Thinking that way, you probably won't take totally away your support from the system, allowing/forcing it to adapt, to change prices and variety, at the same time valuing free webcomics.
Anyway, the law only seems annoyed when money is involved. Cultural rescue websites like Acervo Origens* are usually well regarded, cause there ain't much economic potencial for that stuff.
(May 18th update: Acervo Origens had received financial support from the Government of the Federal District for six months. Now it decided that Acervo was infringing copyright. So everything uploaded at that time will be removed. What a plot twist.).
In a similar way, scanlations don't bother anyone when those comics aren't commercially available (even though it may allegedly take away support from the system). An example is that many scanlation groups had a healthy relationship with publishers, some scanlators being later hired. If a cultural rescue of things that are out of print, lost in time (vertical rescue), is justifiable, then why not a horizontal rescue, between countries and cultures? It's YOUR culture, and you may have some right to share it!
It's okay to buy music from other countries, people do listen to songs in other languages, no problem. But translation is necessary in comics and movies, working as a horizontal rescue. Not to metion import taxes that vary between countries, but can be quite prohibitive. Online MP3 sales cover that spot, but there doesn't seem to be a parallel sotution in comics.
Another interesting point: if, for working with things that have no official distribuition in certain countries, scanlations can be considered a legally grey area, what if we think of English as a global language, that area would be even GREYER! Could that thinking be extended to official online distribuition?
At last, scanlations/fansubs are possible as a way of protest. For example, Ghost in the Shell has had questionable quality translation in Brazil, disrespecting the customers. A good fansub would be a way to protest.
Anyway, do it or don't do it, but think about the world vision that best pleases you, and how far are you willing to commit to it.