I feel like any online discussion forum without active moderation [to force comments to be on-topic] will inevitably have threads that stray off-topic. If a particular comment irks you, at least you have the downvote option.<p>I do think off-topic comments can still contribute valuable information and insight provided they're not <i>unreasonably</i> off-topic. Tangential conversations, as krapp mentioned... everyone is going to approach a topic from their own unique perspective.
Threaded commenting systems encourage tangential conversations, so each comment becomes in essence the root of its own thread, and the deeper you go, the more the context deviates from the original topic.<p>Also, due to /comments, it's entirely possible to start a thread from somewhere in the middle rather than the top.
If you assume they are so off topic what makes you think you'll get answers here on topic?<p>Sorry, that was off topic. I think one of the reasons is the points system. You can still get upvotes/karma for comments off topic. And it is easier to write something when you're not restricted to a topic.
Because often the most interesting contribution is something off-topic.<p>If that were the case, I'd still make the contribution. I don't blame those that do.