Now, I have used it a couple of times already, but not for spam. One time I used it for an accidental duplicate For Sale thread. The guy created a duplicate because his original said "Ship to: Anywhere", but he actually wanted it to say "US48". I saw it, so I marked the original as Held with an explanation saying that I wanted to get it out of public view as quickly as possible for the seller's benefit. I didn't receive any infractions, no warning, not even a private message like "Thank you, but don't do that again".
So I mean, it's a cool feature for someone who wants to use it wisely like that, but man.... I dunno. There ought to be some kind of description or warning in there near the "Mark as Held" option.