I used to use them by preference, the cost was minimal, it wasn't hard to string enough together to make a tour, the buildings were often very individual and sometimes quirky, the hosts were mostly pleasant, the facilities (Laundry, drying room, kitchen, bike storage) were often better than any alternative...
it changed with the Foot & Mouth closures, I know it was struggling before, but that disaster caused a complete re-think. If you look at the map now, there's clusters in popular tourist places, the National Parks, historic cities... and not much between. I gave up about ten years ago, it became impossible to link enough together, the prices shot up, the exclusive whole hostel booking made many unavailable to individuals. I could mix it up, but to me it makes less sense, if I'm carrying camping gear I may as well use it.
Occasionally I'll stay in a YHA as part of a group, on a fixed base long weekend with a club. On my own I'll either camp, or hotel (Usually Tavelodge or Premier Inn), though I have looked at some private YHA rooms, where I'd maybe treat it as a hotel. I've tried a few independents, they vary between fantastic and dreadful, unless someone has given me a recommendation, I won't take the chance.