People would not follow a leader if they felt that there was nothing they could gain by following that leader.
Conclusion (therefore)
Even incompetent or evil leaders bring some good to their followers.
Evidence
If people felt there was nothing to gain from following a leader, they wouldn't follow them.
Evaluate
By contrapositive, the premise is saying .
So we know that people who followed an evil or incompetent leader felt there was something to gain by following them.
But the conclusion is claiming that there was something gained by following them.
So this is a variation of the famous Intent vs. Result flaw (or Belief vs. Fact) People intended to gain something by following a leader, but whether that resulted in gain is another question.
Goal
The essence of that flaw is, . So we can look for arguments vulnerable to a similar objection.
If the answer were going to mirror the structure more, then we'd have an argument like,
Prem: if ppl didn't believe X, then wouldn't do Y. Conc: So when it comes to people who did Y, we know X is true.