I'm in the same camp as John P - spotting anachronistic tunes makes me a bit smug, pleased that I'm educated enough to know the difference. I'm also realistic enough to know nobody much cares other than other trad enthusiasts....
Except...
There is one shining example of consequences hitting hard. The film makers of "Napoleon Dynamite" realized a bit too late that "Music for a found Harmonium " wasn't trad, and taking it from an album recently produced w/out permission had even more costly consequences.

Taking the smug knowledge and making it useful is sometimes possible. A dance band I'm in plays some benefit gigs for the (non-folkie) general public. When someone comments that Ashokan Farewell is a civil war tune, I enjoy buttering up my correction of that by describing Ashokan, the setting, how enjoyable the music camp is, and one's reluctance to leave such a great experience, the source of the tune's inspiration, and how that place rings with traditional tunes year round.

Joanne in Cleveland