I think it's a question of how you see the debate. What it is, or should be. Is it between the two candidates, and moderators merely give it structure? Or is it a debate with an expectation of truth and trustworthiness, fulfilling the press code, where the moderators would have to at least point out lies or ask for clarifications?
A debate between two candidates has its value, but we can't deny it strengthens Trumps position as an apparently to many people charismatic liar. Between only two people it's about who is more charismatic and convincing, not about truthfulness, verifiability. All of those only go as far as the other candidate can establish them.
If many citizens watch only the debate, is that enough to inform them / base their voting [or omission thereof] on?
In the end, it may be understandable to wish for moderators to point out lies. It can be irritating and frustrating to see lies on a podium finding success, without successful, conclusive rebuttal. But that's not the moderators' place in the show format as it is.