I hadn't heard that thistles like compacted soil, here, they seem to like very rich non compacted soil, old corrals, fencelines, tree lines, but don't do so well in the wide open hard fields, but looking at pictures of their root mass online just now, they have to do some good for compaction and OM.  

I converted some old corrals to cropland, and am still dealing with the thistles.  The first year I told my neighbor that it was sure a good thing I planted all of the thistles to help keep the barley standing up, since where there were none, it was lodged flat, not sure he bought my story though.