What's so bad about a comment going unseen?
The fact is that most comments go unseen. It's a big world, and it's very rare that any of us has a truly unique fact or viewpoint to share. Sometimes people will see your comment and vote or engage in conversation, which is great, but it's very hard to tell which comments will attract attention and which won't.
And that's OK. It cost you nothing to make the post, and it probably didn't even cost much of your time. From that standpoint, it's pretty much always "worth it".
It does cost a certain amount of additional clutter in the universe: your comment is probably redundant with others, and it's probably not the best written of those. But your comment probably won't be the one that pushes a thread over into unreadability.
If you comment a few times a day, write politely and clearly, and are actually writing with the intent of communicating rather than simply talking to hear yourself talk, then pretty much anything is "worth it".
But if your metric of merit means that people see it (and indicate that they've seen it by voting or commenting)... it's pretty much never worth it.
