I think that the 3399 is probably the hook that most winged wet fly tyers have used in the past; and that's probably the reason that it still has some preference. Frankly, you can tie most flies on any hook that you want; as long as you're willing to change the proportions on the pattern to match the physical size and shape limitations of your hook.
There are some winged wets that I think look better on longer hooks; particularly those that have a lot going on with the body; such as the Royal Coachman.
The S70 3399's are fine as far as I'm concerned and I'm not certain about this, but someone told me that Mustad had either discontinued or was planning to stop making the straight 3399 hook. Too bad if it's true.
When tying flies for fishing crosses the boundary and flops over into tying flies for art, then exact proportions become important to some people, since duplication of the original pattern is usually the goal; something for which I'm not sure that I've ever firmly grasped the rationale.
From my standpoint, if it catches fish for you, then you've succeeded as a fly tyer.