Gwenorgan gave one good answer. You can always do it yourself or with a smaller group if you're past the nominal difficulty of the dungeon. It won't give you as much in points, but you get a much larger share of the loot. Too often when you run a pickup dungeon with a experienced guide, you wind up deliberately skirting monster groups that are off the ideal path, some of which can drop worthwhile loot. If you have a gathering profession, you might have to forego valuable skins or herbs or minerals to stay with the group. It sucks having to pass a pile of dead wargs or dragons and their 5GP hides. Soloing it or working it with your own group gives you time to poke your head into interesting nooks and crannies.
I used to be big on the completion thing - but all it wound up doing was making my quest logs clumsy and unmanageable. I've swung almost the other way now, when leveling, anyway. Once a quest or dungeon goes green, I nearly always abandon it and move along to more profitable pastures.
Another reason I sometimes solo low level dungeons is that I might want or have to be a dungeon guide one day, and soloing gives me a much better chance to learn the different dungeons for later.