since that technically saves a bit of storage, however little
Perhaps this is a side-effect of separation between the administration and moderation that happens on this site - none of the mods have anything to do with the storage available and have never been informed that this is the issue.
In general, I wouldn't object to somebody thread necro'ing for a valid reason (although may have done in the past
). Problem is, that generally thread necroing also involves a change of topic, a separate (but related) question asked after 2 pages of post, does not "flow" from the original topic, or is simply a "me too!"-type post.
My current reaction is generally to simply lock the topic (often with an explanation, but not necessarily if it's just a "me too!") or, if the new question contains enough background information, I try to split the topic.