One thing in favor of keeping them is that sometimes having an anchor point for the research into those parents, and specifically space to add additional children of theirs, can result in finding those siblings listing one of your family members as a niece or nephew on a census. I usually keep them marked Uncertain, but don't remove them and instead use research notes on their profile to store all of my working theories. I can tweak them as needed if they're not-quite-right based on the research, and if I do end up having to remove them as the parents, at least there will be some detail in there, they won't just be a couple of names, and maybe they'll find the rest of their family.