It's true there are greater journalistic crimes than plagiarism. When a reporter fabricates stories, or passes along government lies as truth, people can get killed. Plagiarism has never started a war, as far as I can tell.
But that doesn't mean that it's not a serious matter, at least for the journalistic community. It's a matter of workplace theft.
Imagine that you wait tables in a restaurant, and one of your co-workers turns out to have a habit of picking up other people's tips when he thinks no one else is looking. It may not be the world's biggest crime–but it's probably going to matter to you and the rest of the waitstaff, and it's probably going to get this waiter fired. If it doesn't–if the boss looks the other way because he's popular with customers, say–you and your fellow employees are entitled to feel betrayed.
When a writer lifts other writers' words without attribution, or pretends to have done reporting that another reporter actually did, it is viewed as stealing. And if editors don't take it seriously, we're collaborators in the crime, which is something we don't want to be.
It doesn't affect readers directly–but if you want journalism that you can trust, you need writers and editors who can trust each other.