Where posting “flagged” can become a problem is when the question may actually not be spam. Granted that there are Qs that are blatantly spam. But there are also many Qs that we’re pretty damned sure are spam (or will turn into spam at some point), but we can’t be certain.
Yes, long-time members can get pretty wise to the ways of spammers and may have a highly-developed “spammy sense”, but it’s better to give Qs that fall into the gray area the benefit of a doubt. Sure, flag the question you may be suspicious about; the mods have tools for diagnosing spam that members don’t have, and we’ll check it out. But posting “flagged” on a suspicious Q in effect accuses the asker of breaking the rules and discourages anyone else from answering the Q. That would be a real shame if the Q is actually legit.
So my personal stance on the matter is that it’s safer to flag but not post any comment at all. That may indeed result in several flags on the Q since no one knows that it has already been flagged, but as I said above, those multiple flags may actually help draw our attention to problem Qs. That said, if there’s a Q that is unambiguously spam and someone posts a “flagged” comment on it, I don’t see that as a big deal.