I know it's common on other sites, and that's why I don't frequent those sites. But it isn't really allowed here. If you see a nasty comment, especially one attacking the poster's views rather than posting their own, report it. Admin doesn't read every post, and when you report it, that allows them to view it and make sure it's allowed...or edit it.
This is a mental health support site. With that said, it's necessary to remember that people who post here have to take extra effort to give a good response, as they are ailing themselves. I think the membership does a pretty good job in all. A few members struggle more than others at times though.
Also, remember it's what state of mind the reader is in too, as to how the post is taken. Sometimes the reader ends up reading between the lines and fully misunderstanding the intent of the poster.