The assumption that leads to our miscommunication is that you don't consider how much stronger 78s is than 78o WHEN IT DOES HIT/PROVIDE A STRONG DRAW. The reason I said you are too preoccupied with pre-flop/showdown values (which amount to the same thing as the only fixed info we have on the strength of hands pre-flop is how they show down).

So, what I am saying is that although, at showdown, a flush draw with 78s will lose to ace high, DURING the hand we can play that same draw as if it were a strong made hand, thereby winning lots of pots through the villain folding or through our draw hitting. Obviously if all we did was checked or called, you would be correct - the value of the hand would be very low. But it's because we can be so aggressive with it (and because it can win stacks) that it is such a favourable hand to play.