What's the problem with commenting on an old thread?
The one that prompted my post was a question posted over two years ago, to which the last post was also over two years ago. Another user, for reasons known only to him, replied to the thread this morning with an alternate way of addressing the OP's issue. The OP posted two years ago--either he's already figured it out, or he's given up. Another answer really doesn't add anything to the question, and it doesn't really help other users searching for answers, as the new response isn't related to the subject of that thread.
Tagging on to an existing, truly relevant, thread with one's own question is a different story, but I don't see that happen very often. I'll see it when the original thread is from two major versions ago, so the answer (and thus, really, the question) is completely different. More often yet, the question isn't even very closely related to the earlier thread.
You're right, though, that it indicates the poster has searched, and that's a good thing. But IME, most replies to long-dormant threads belong better in their own threads, or simply not posted at all.