This functionality might be here already but if so, I could not find it. If not, I feel that sometimes you want to ask people questions or defend a question. Without spamming the comments, would it be worth having the ability to send private messages?
You can. Click on the chat link in the top menu of links. It's not private but you can interact with users real time. Otherwise comments are the only way.
Any other forms of private messaging has always been declined, and this would be no exception, especially with the chat officially available.
Sometimes, you can contact a user by checking their profile to see if they have contact info in their description, or a website that has contact info.