Just a quick note about my background before I wade in here. Had been a sea kayaker for almost 10 years before getting into the racing scene by accident (was asked to do the paddle section of a multisport team race by a friend). Had been paddling with a Greenland blade and a small asymetric conventional blade up until then. Got hooked on racing and have been doing a lot more of that over the past 4 years. Around this time I started using a wing. I now use a Lettman 1 blade for sea kayaking and the ski and a Jantex Gamma for the K1.
I now paddle a sea kayak, a surf ski, and a flatwater K1. And regardless of which kayak I paddle I will always use a wing blade. The efficiency and power of a wing cannot be matched by a conventional paddle of the same size. Sure, you cannot do some strokes with a wing. But since the vast majority of one's strokes in a kayak should be the forward stroke I don't consider that a problem. In addition, the wing gives you great lift for a roll and you certainly appreciate the extra power in a headwind when this may become a safety issue.
What many sea kayakers don't realise when they criticise the wing blade is that there are two main types - the teardrop and parallel edged. The teardrop (i.e. my Jantex blade) is what most flatwater racers use. It has a powerful catch and exits quickly. They're great on flatwater but can be a bit unpredictable when it starts to get rough. The parallel edged blades (i.e. my Lettman) are very smooth throughout their stroke and are more predictable in rough water. Most parallel blades can be used with a high or low angle style, unlike the the teardrop style. The main issue is that using a wing does require a different style which many users of a conventional blade initially find difficult to adapt to. Try to use a wing like a conventional blade and you'll probably end up swimming. It took me several months to make the transition.
Virtually all surf ski paddlers use wing blades and many of these guys paddle water far rougher than most sea kayakers will (on much skinnier boats I might add!). I understand Freya Hoffmeister used a wing blade exclusively on her recent circumnavigation of Australia. In my opinion, if you are interested in covering long distances efficiently then wing blades are certainly worth some serious thought.
As for crankshafts with a wing, I'm only aware of one manufacturer which offers this (Lendal). I personally don't think you need a crankshaft with a wing unless you have wrist problems. And if you have wrist problems (or elbow problems for that matter) when paddling then perhaps you need to have someone look at your technique.