I like the Question because of Batman/Huntress: Cry for Blood, the JLU animated series, 52, and The Question: Pipeline. I like Nightshade because of Suicide Squad. I like Blue Beetle because of Birds of Prey. So that's my reasoning.
I'm not sure we can really say they can't hold a solo - they've not been tried in a really long time. But I don't necessarily want a solo - I just want the richness of the DCU history to be used more in supporting roles.