I've not ever tried it for templates. Usually, I use the NON MELT template plastic sheets from Joannes or Clotilde. ASK ME HOW I KNOW I NEED THE NON-MELT KIND!!! lol.
For templates that I know I'm not going to iron around a whole lot, I cut up the plastic sheets from bacon, plastic margarine covers, anything stiff enough, but flexible.
The foam core board. Hum. Depending on which style it is, I can think of a few uses... like under my sewing machine or serger. 1 to raise it up just a bit higher so that I'm closer to my work on the machine bed if I'm going to be sewing intricate points or something. or 2. under the serger to cut down on the vibration/noise sergers make.
I suppose you could also pin block pieces to it, to try out placement of fabrics used, contrast, one-ways, etc. Just to see how the finished block will look best sewn together. Like a mini, on the table, display board.
You could cover it with scrap fabric and make yourself a pin board so you can pin up your quilt pattern/instructions for easy read as you go along. lol. I use a lucite cookbook/photo stand for this. Comes in so handy if my pattern is photo copied or torn out of a flimsy magazine or something.
But no, never tried foam core for templates. And I don't think I would.