You’re correct that the Skagit River is the area more popular for the "purists". I've always found that I'm more successful and enjoy flyfishing lakes more than rivers. I sometimes use my 8wt to fish for pinks and cohos but mostly use gear to river fish (with the exception of non coastal rivers where I always use the fly rod). I couldn’t imagine being able to land a spring on the fly, certainly not with my setup.
On that note one of my fishing partners uses the fly exclusively for salmon (I have seen him hook into a spring but he was unable to land it, and he seems to be able to hold his own chum fishing).
Sandhead, I can see fly fishing working for Salmon, if the waters are calm. Not on Vedder right now, since you cannot get the fly to the bottom.
Also know of people who ff in the ocean close to Cap's mouth. But I am told you need to cast really good to do that (I am working on it). I don't have the proper set up yet (I have a 6 weight setup), but can see myself getting into 9 weight when I am better. I believe there will be some fall salmon fishing (Mamquam?) that would yield itself to ff.
As far as Skagit River, I am told is a slower river with deep pools, etc. I personally like the challenge of navigating rocks, throwing the line with some precision, etc. which is lacking from deeper lake fishing. But evernone has their own preference