If all you care about is benchmark results, there's an obvious incentive for the drive to claim falsely that some data has been physically written to disk.
On the other hand, there's far less incentive to lie about barriers. If all you're saying is that '*if and when* this data has been written, then that other data will have also been written', you can still happily claim that it's all done when it isn't, without breaking the barrier commitment.
When you have that commitment, it's possible to build a transaction system upon it which works even under the assumption that the drive will lie. You're not going to achieve the full benchmark speed, but it's going to be far better than turning off the cache.
Of course, whether drive manufacturers see it that way is another matter. Is there any data on whether drives actually honour write barriers? It would be interesting to see if there are indeed drives that aren't expensive enough to report accurately on when data has been written, but still honour the barrier request.