I'd say it depends on what the new reply is doing. If it adds new context, clarifies old context, or updates the original thread, more power to it.What bothers me is usually when someone responds to the old thread, the snide remark that generally is quick to respond. It's one thing to point out that the thread is old if it was a time sensitive issue but I typically feel the remarks are geared toward the rude side when pointing out the thread is old.
If, however, it answers a question posed by a poster who hasn't been seen here in more than a year, I'll roll my eyes... literally where I am, and figuratively in the same said thread.