I don't have any experience with Usenet or any current DDL sources. I generally check public trackers first, then private trackers, and mostly use private trackers when I can't find something on public trackers.
My selfish reasoning is that with public trackers, I don't have to worry about my seed ratio, etc. I generally seed to a 3.0 ratio, and I do have some older/more niche stuff set to infinity, but when I need to clear some space in my seed box, it's nice to be able to delete stuff without worrying I'll be penalized.
From a less selfish perspective, if it exists in decent quality on public trackers, I'd rather contribute to keeping it alive there, where more people can access it.