I sort of like the idea, at least the option. Having this kind of memory cache would, if nothing else, help with flaky memory cards that are occasionally slow to respond, causing errors.
The problem is that RAM is completely volatile, i.e. if the power goes off or the camera fails all data is lost. This means you would be bound to using a camera with a battery rather than a capacitor, and many of us have seen batteries are problematic even with current modes of operation. Having to transfer the previous 15 minutes of video if power goes off would be harder, and even more prone to failure. And if the camera glitches and locks up for any reason, again, all data gone.
You could emulate your alternative way of working by having constant recording, but instantly overwriting old unwanted footage. That would be secure (from a privacy sense), but stress the memory card even more. Or delete the old unwanted footage and continue recording on a different part of the card, that would be kinder to the card, but deleted data could be recovered using software (so not securely deleted.)
So I think the main benefit would be ease of reviewing data. You can do that in other ways, e.g. when I review video I only look at files with the Read-Only flag set or ones placed in an event folder.