For some reason, scanlations have always bothered us greatly, and thinking about it, I can come up with more than one reason why this might be the case. First of all, they're called "scanlations." Obviously a much better splicing of "scanned" and "translations" would be "scanslations," with an S, because it rhymes better, and is easier to pronounce. For some reason, the NL combination is actually pretty hard to say, but if you stick an S in the middle, it's much easier. How "scanlation" without the S came to be mainstream, I have no idea, but it has always been a not-so-minor pet peeve.
Now, I could say that the reason we hate scanslations, now that I've determined that that's what they should be called, is for the same reason we're not so fond of music downloading. We just like to buy our music and manga, because... it makes us feel special or something. And because, despite having no qualms about downloading entire episodes of anime, we'd feel bad downloading a ton of music or manga. Our justification about anime is that most anime can be viewed on TV in Japan, and so Japanese people don't have to pay extra for it. In that same vein, it could be argued that, if you were in Japan, you could theoretically stand in the bookstore reading manga (just not in US Kinokuniya stores, because they shrinkwrap everything).
It has been argued that the advantage of scanslations is that American fans get to read the manga much sooner than they normally would otherwise, so I did wonder for a while if it was unfair of me, being able to read Japanese, to expect fans to wait for legal translations. But it's not like we grew up being able to read Japanese--we worked for it. And we didn't download the raw (though techinically, it's not like a scanslation has subtitles, so raw probably isn't an accurate term...or is it? Anyway...) Ichigo Mashimaro manga from l33t-raws, much as we would love to read it. That was an awesome anime.
Still, it's really none of my business whether people read scanslations or not. Although I could play the card that says that, as a professional translator of manga, it is my business, because people not buying domestic manga because of reading scanslations would mean our clients get less money, and therefore can't pay me as much. But I really don't think that scanslations are affecting the industry that much currently.
So mostly our problem with scanslations is this: Since we ourselves choose not to read scanslations, it's very frustrating when everyone else goes out and reads them and gets farther in the story. So basically it's just petty jealousy. That, and our anti-spoiler Nazism coupled with a fear that others might slip and let something out.
For example, when we had only seen half of the Fruits Basket anime, and we weren't translating the manga yet, some girls were oohing and aahing over a Kyo bracelet someone had, and one of them said they'd hate to be around when he took it off. Though I guess it wouldn't be a problem if those kinds of things weren't exactly the kinds of things that sound like spoilers and therefore make themselves harder to forget. And we tend to remember things anyway, and then think too hard about them and draw conclusions.
There is one thing that bothers me, though. I know of some artists who insist on being paid for their art, and sometimes only do art when they know they'll get something out of it, and then go on to download scanslations. Isn't that a little hypocritical? Manga is a form of art, and manga artists deserve to get paid for it just as much as fan artists, if not moreso.