hairy - I'm a little baffled that no one has taken a crack at this. Maybe its a bit basic for those who have grown up with Waddingtons. I fish salmon mostly in Canada - though I have been coming to Scotland in the spring, and will be again in 2009. It was in Scotland that I learned about the fly. Waddington himself theorized that by the design of the salmon's mouth it fed on relatively long, slim baitfish like squid and sandlance. The extend length of the Waddington shank allowed him to tie a long fly and locate a hook at the end of it to stop short strikers. Also, the hook to shank joint is fairly flexible - I actually use Partridge big mouth doubles connected to the shank with spectra line - and so you do not have the likelihood of your hook being pryed out of the fish as you would with a long shanked hook.
There are certain flies that really lend themselves to this design. The Willy Gunn, Allys Shrimp, and long, hairy flies like that are the ones. I think the long slim profiles are attractive to fish in high and/or cold water and produces more strikes than shorter flies.
One might ask what the benefit of the Waddington is compared to a tube, and my view would be first in the top and bottom orientation maintained by the front up eye design of the shank, and the fact that the material is tied to the shank like a regular fly. A tube can spin around and lose this orientation. Also, long tubes, especially ones of heavy material become a little unwieldly. You can extend a tube with the plastic junction tubing, but it doesn't tend to stay as straight and that can cause twists when in the current.
I would suggest the lack of replies is because no-one can think of a good answer, myself included.
A fly tied on a Waddington shank can vary in length from under 1" to over 3" and would fall somewhere between an aluminium and copper slipstream tube in terms of weight. As such if you wished you could use them at any time of the year and at any water height.
At no time in a season do I feel compelled to use Waddys, infact I never use them prefering tubes instead. Some people dont like spinning with Tobies because they think the fish can lever the hook out due to the articulation between the lure and the hook, I feel the same about Waddingtons shanks, hence my preference for tubes over Waddington shanks.
I use them, leaverage is not a problem as i use a bit of shrink tube over the hook connection retaining flexibilty, this means i can change a hook if required though it is too fiddly for the bank.
Metal framed waddingtons replace light snake flies when sea trouting in faster / deeper runs. yet retain a slimmer appearance than tubes, though i also use tubes.
Waddies have become fairly popular here in the PNW for all the reasons noted above. The one major difference is few folks actually use a hook connected in the lower ring.
'The Norm' is to rig them (hook wise) the same way you'd do a tube fly: leader through the upper eye, down the side of the Waddie, through a bit of tubing, tie on hook and slide hook eye/tube over the bottom ring.
As the leader DOES NOT go through the lower ring, the fly will pop off and slide up the leader with 'the take' ala a tube fly.
I don't think that there is any situation where I have ever felt compelled to use a Waddington. There are times though when I have felt that perhaps a Waddington might swim better or simply behave better in the flow I am fishing than a tube alternative. There were times though when I have lost a fish on a Waddington, whereas if I had been using a tube I have felt I would have landed it. I used to fish Waddingtons with a soft length of rubber, or silicon tubing to hold the treble in line with the shank. Due to the articulation of this method though I felt the pivoting action of the hook once a fish took, was never sustaining constant pressure on the business end of the hooks. I now use two methods of attaching the treble. In the first instance I use heat shrink to hold the hook straight. This once shrunk gives a firm inflexable connection and keeps the hook straight in line with the shank at all times, even when playing a fish. Sometimes though fish will chew on the fly while being played and as a result waste it. Another alternative is to attach a treble in the same fashion as you would a treble to a tube. I pass the nylon through the front eye of the shank, then through a piece of tubing and then tie on the treble. If I am fishing a strong flow though I will also pass the nylon through the rear eye of the waddington, before then passing it through the tubing and tying on the treble. With this method the hook comes free of the shank while the fish is being played. It also gives a better hold on the fish, plus your flies dont get wasted.