Do you think that 12-marker matches are worth following up?
I rarely look at them.
Yes, definitely, especially if the surname is yours. I found a second cousin once removed via an exact 12-marker match.
I should expand on that somewhat by saying that my second cousin subsequently upgraded to 37 markers and that our exact match held up at 37. I wish he would upgrade some more, but he isn't as interested as I am.
Anyway, I was able to find his entry in the family book written by one of my paternal great grandaunts, which confirmed the y-dna match with paper trail info.