I agree, good question!
Do jet-powered craft compromise the "esthetics" of flyfishing??
Yes, in my opinion they do!! I much prefer to walk or float in.
That being said, I should list a few other things that mess with my flyfishing Fung Shui . . .
1. Walking or floating a distance to get to a good run only to
have a jetboater beat me to it.
2. Lots of other anglers infesting all the holes I want to fish
3. Getting lowholed in a good run by another fisherperson
I personally fell that, if you're planning to fish the Pugetropolis rivers, you need to be ready to deal with the reality of fishing them (see above). A jetboat helps even the odds when fishing on crowded waters.
Many of the finer "esthetics" of flyfishing are already lost - you're going to deal with (other) jetboats, lowholers, and crowds anyway so it's reduced to determining what I want to get out of my fishing time. For me, it's the opportunity to fish as much quality water as possible. The jetboat's speed and mobility provide this.
If you want a wilderness experience, there are plenty of places to hike in. Head up into the upper Hoh in the park or other remote stretches and you won't have to deal with jetboats, driftboats, or (hardly) any other anglers. I like these excursions best, but they take time and I can't always invest a weekend to really get ~AWAY~. And, as Kush mentioned, there are also those wilderness rivers with minimal launches where the options are a multi-day float, a helicopter, or a jetboat. And Ol' Steelhead Bob explored some pretty wild BC headwaters for steelhead in his jetraft!
Fishing from jetboats are part of a compromise I make to achieve as "quality" a fishing experience as I can find on certain of our more crowded and accessible rivers. Not a perfect solution by any means.