Personally, I believe that you can't just add up mildly bad things and create a very bad thing. For example, I'd rather get my finger pricked by a needle once a day for the rest of my life than have somebody amputate my legs without anesthesia just once, even though the "cumulative pain" of the former choice might be higher than that of the latter.
Having said that, I also believe that there is sufficient evidence that shrimp suffer greatly when they are killed in the manner described in the article, and that it is worthwhile to prevent that suffering.