It depends on what your definition of "efficacy" and "results" are.
If all you want is to collect data on the cosmos for the sake of pure science, then absolutely human space flight is a waste of money.
If your goal is to inspire, create an atmosphere that instills interest in a space program (of all types, including manned and unmanned), promotes STEM fields, and is a jobs program, then you'd probably find it very useful and cost effective to sink money into a human spaceflight campaign.
Guess which one of those goals is NASA's true mission.
These sorts of conversations so often ignore that NASA is an organization controlled by politicians and is ultimately responsible to them and the US public who elects them. It's not the engineers and scientists employed or using the data that NASA creates. At the end of the day, those politicians want to have their name connected with inspiration, not just data on the water content of mars.
How many kids do you know that say they want to be an unmanned rover on mars when they grow up? How often do you hear JFK saying "We go to the moon, not because it's hard, but because sending rovers is really cheap and we'll get a much better bang for our buck." How often do you hear someone replaying "Surveyor 1 has landed, one small step for a robot, one giant leap for mankind."
You don't. And there's a reason for that.
You don't have to like it, but you also can't ignore that in reality, NASA has goals that are not just about the science.