I guess I don't get that, while yes, it would be more expensive to shingle a purlin than a rafter roof because you would have to first board over the purlins and then apply skip for the shingles, it would require no fewer layers to do board roofing over purlins - Though you can do board roofing over common rafters with no sheathing whatever, doesn't get any cheaper than that.
And it doesn't explain the regional dominance Common Purlins hold in much of Northern New England. Just across the river from where that 1715 barn stood you're in common purlin land, which is why I have a curiousity for how common they were on the south bank.
And as I have said in other threads, they are (and were hereabouts for a good 200 yrs) a perfect marriage with ETJ's - No load path whatever for thrust to reach the Plates.
