I don't think its that there is anything inherently bad with reviving an old thread..
My personal take is that the object is mainly to alert people that they are about to post in a thread where the original participants may have moved on long ago -- that the discussion they are replying to may contain out of date information and opinions, and that others on the forum are probably not following the thread the way they would a more recent thread.
So the warning is to catch the attention of the poster and give them an opportunity to re-frame their intended post in light of the fact that it's not currently an active discussion.