I have finally reached 10k on stackoverflow and since then got into the habit of taking a look at the flagged posts every once in a while.
A surprising and recurring theme among posts flagged as spam is that many of those aren't spam, at least according to my definition which isn't very strict, but which would include a link in a flagged post. It's similar with posts flagged as offensive when all they are may just be an insult to someone's intellect. Posts that are mere junk get flagged, sometimes also stuff that people think is so bad, it deserves some penalty.
Apparently, people hesitate to flag for moderator attention, I guess because they don't want to bother a human being that may have better things to do than deal with useless junk posts. Thus they flag those useless posts as spam.
A post here on meta asked for another flag for exactly those kinds of posts and the accepted (and most upvoted answer) said you should flag for moderator attention. Alas, this doesn't seem to cut it.
I think the fact that we don't have other flag reasons (besides "offensive") is also responsible for the ridiculously tiny amount of spam flags users have per day.
Would adding another flag help? Or should the reputation threshold be raised that enables you to flag? Or is there another option?