I agree that diagnosing others (online or otherwise) is undesirable, especially based on limited information; in everyday life it's often just people's way to slap a label on you so they can dismiss you as crazy. "Self-diagnosis" is also potentially dangerous, but on the other hand, it's nearly unavoidable to at least form a suspicion when you're experiencing symptoms for so long. To some extent maybe that's good, otherwise many people might not even go to see a Pdoc in the first place.

I had a pretty good idea that I might be bipolar before I went to my Pdoc for the first time, but it took me years to come to that realization. Until then I just tried to brush off my symptoms and move on (which proved impossible). If I hadn't "self-diagnosed" to an extent, I might have never seeked help. Not to mention that where I live, you need a referral from your GP to even get access to a Pdoc, and most GPs suck at diagnosing mental illnesses. If you don't tell them what you think you have, they will just send you home and tell you to "get plenty of rest."

The thing is to always keep in mind that you could be wrong, watch out for confirmation bias, and be careful that you don't selectively give information to your Pdoc to steer them towards a given conclusion. I tried to collect as much objective information as I could (like family history and mood charts) and let my Pdoc make the assessment based on that.

What I'm saying is that "self-diagnosis" can be a crucial step towards treatment, but should be treated as nothing more than a layman's first impression.