I've heard mention of this, too. I think a lot of people had trouble finding a source of clean pine needles. I haven't tried them, even though there's a big dead pine tree still covered with needles behind my house. I'd think the main motivation would be to provide acidity to the potting mix, but 50:50 peat:coarse sand with a splash of vinegar once in a while seems to work well for me. I'm about the opposite of an expert, though, so take anything I say with a grain of salt, or sand, or whatever...