I would agree wading isn't necessary all the time, but its can be advantageous much of the time.
Totally beach dependant of course, as well as the ability of the caster, but coho are most likely to be caught in the rips where the bait is generally pushed, and on many beaches wading out to reach this water is often the way to go.
I've caught coho with dry feet, but I've caught WAY more coho waded out to maybe my knees or mid thigh. A lot of good coho beaches are flat for a decent ways out before dropping off. Since these drop offs, often focused around points, are what helps create the rips, getting out a bit often helps.
Of course there are many variables at play, and fish will come right up to the beach at times, but usually only if they have a reason to....I.E. food. I find with coho that the food isn't often tight to the shore, so I generally don't feel I'm missing much by wading out a bit.
Of course there is such a thing as wading too deep. Herring Dan was the king of this. Standing chest deep right smack in the rip was his signature move.
As with all of Puget Sound fishing there are very few black and white rules, so I'd always encourage anyone to do what works best for them. With the constant changing environment on the sound it's also important to be aware what's happening around you and adapt accordingly. It's part of what makes the fishing so enjoyable to me.