Sometimes this is true, but sometimes undertrained perspectives can be quite detrimental. The most clear-cut example is that of medical discussions; medical commentary or speculation from under-qualified individuals can do real harm. Many communities on the web have recognized this and have taken measures to protect users[1].
Medical advice is a pretty extreme example as far as detrimental discussions, but there are some shades of gray that I've seen show up on HN. As a biologist, the ones I've found most grating are those that reflect poor training in the the life sciences, where HNers make pseudoscientific or unscientific claims. It's easy to call these things "opinions", but they are actively harmful when most users will not be able to tell that they are incorrect.
There are certainly other cases where underinformed commenters end up doing harm, from legal speculation to "get your pitchforks" outrage attacks.
Note that I'm not arguing against free and open discussion in forums such as HN; people should be physically able to post whatever they want. My point is instead that the community should be sensitive to underqualified commentary in disciplines beyond its expertise. Less accepting, in a way.
Credentialing is but one way this has been done, but understandably not the best one here.
[1]http://www.reddit.com/r/askscience/comments/s4chc/meta_medic...