I know Ford's official position is that this doesn't lead to any other issues or lead to other damage to the engine, and I think in the time-frame that Ford's most interested in (how long can we get them to wait while we get parts in and a slot scheduled for them without needing to provide a rental) there won't likely be any damage. I also think it's unlikely to break the timing chain or have it come off. Keep in mind that as long as the tensioners are keeping the correct pressure on the chain then the part making the noise is the center part of the VVT gear. The outside, meaning everything the timing chain touches continues to move as normal, in total sync with the crankshaft. The center part of the VVT gear, though is getting slammed back and forth and your cam shaft is moving with it, meaning the valve timing will be off and will look nothing like what the computer is commanding while the clatter takes place. Eventually some of these engines with bad phasers will develop a warm-idle knock, apparently meaning at low engine speed (low oil pressure) a little bit of uncommanded movement of the VVT system is occurring, which means the computer wants something that the mechanics of the engine can't deliver. At that point you can get codes thrown related to timing. I would think also that if you go a long time that extra movement in the VVT sprocket will cause more wear there and make the components in the VVT sprocket fit more loosely. This will allow more oil to escape the VVT unit and make it harder for the delivered timing to match the commanded timing.
My personal experience, I lived with my phases for probably 3 months after the startup clatter got pretty bad while waiting on parts, and the warm idle knock became more noticeable and I eventually got a check engine light--strange, it said lean bank 2. That's when I fixed the phasers. All sounds including the warm-idle knock went away, and the engine code never came back. Short answer, I think eventually you'll have drivability issues in addition to a noisy engine if you let it go for too long unless you're very lucky. Of course, every failure may be a little different, and while I got a few noisy months someone else might get a noisy year.