It's good for confirming someone is giving honest testimony, at the very least.
That all you have to do is not speak to avoid speaking the truth doesn't actually help the person not speaking though - usually a zone of truth is used in a context that refusal to answer a question can be interpreted as a clear answer in itself, i.e. "Did you murder the inn keep?" being followed by silence is just as good to the people asking as if you had said "Yes," because there is literally no reason not to say "No," if it is the truth. Or, to phrase that differently, defeating the mechanics of the spell easily is not the same as defeating the use of the spell easily.