Virgin has indeed always had extra legroom seats. Whether 'Economy Delight' was a way of managing to more easily sell them or not I'm not sure. Guess it packages them together rather than making someone make a conscious decision at a later stage to add them, which may deter them.
I will pay for extra legroom in Economy given the opportunity to do so, but many will not. A recent experience on QR's 777 however is that it is shoulder room and not leg room that is the main problem in many economy classes.