No, not really - they can decide at any time to start storing that metadata of their own will, use it internally, sell it to advertisers or turn it over via a direct law enforcement portal if they want. Their access alone is in many ways a problem.
Of course, we are agreeing. But as the other front page story shows us, even companies with the best of intentions and made up of people with strict moral codes devoted to providing their users a secure product can be usurped by government forces with ulterior motives.
I believe Whisper doesn't want this data. I can even trust that they would never collect it of their own volition. But it's irrelevant if they can simply be compelled to collect it, or even worse, someone within their organization can be compelled to secretly collect it and may even do a better job at this secret collection than Mayer's lackies at Yahoo!.