Some cessationists cite a statement attributed to John Owen. Commenting on Owen, Packer says:
He is quick to deploy against them the old dilemma that if their ‘private revelations’ agree with Scripture, they are needless, and if they disagree, they are false. J. I. Packer, A Quest for Godliness: The Puritan Vision of the Christian Life (Crossway, 1994), 86.
That has epigrammatic clarity and concision. However, it's unclear to what extent Packer is quoting, summarizing, or paraphrasing Owen. As it stands, the statement, while pithy, punchy, and quotable, poses a simplistic and fallacious dilemma.
I assume "agree" is synonymous with "consistent". To "agree with" is to be consistent with. Put another way, to agree with means it doesn't contradict it. If so, the statement is deceptively appealing.
It's like saying, if medical science is consistent with Scripture, then it's needless; and if it's inconsistent with Scripture, it is false. The latter clause is true, but the former is false.
To say something "agrees" with Scripture just means that it's consistent with Scripture. But that doesn't make it redundant. Something can be consistent with Scripture, but add to our fund of knowledge–like medical science.
To take a hypothetical case: suppose I have a premonition or dream that if I board that plane tomorrow, it will crash. I reschedule. The plane I missed explodes in midair, killing everyone on board.
That "private revelation" doesn't contradict anything in Scripture. But it's not superfluous or needless.
The statement attributed to Owen makes the mistake of attempting a quick and easy refutation of a position that isn't that simple. Whatever your position on cessationism, this gambit is a nonstarter.