I think there are a couple of important points:
* There are still elements that VE can't edit;
* There are still pages that render incorrectly in VE;
* There are still likely other bugs, roundtrip and
performance issues that will frustrate users.
We do want to eventually get all users to use VE, but we
need to be realistic that at this point we're launching beta
software that has lots of quirks, some known, some unknown.
It's awesome already but there's still a long way to go.
That means power users will have _legitimate_ reasons to
want to continue to use wikitext for some time; it'll be more
efficient in many cases, and sometimes necessary.
I want us to be careful to not message in a way that
glosses over such concerns.
I'm personally fine with the preference as an option, but
we need to make sure that power users know about it. I can see
the concern about a secondary "edit source" option from a UI
cleanliness perspective, but again, recognizing that VE is
still beta and will be for a while, I think James' argument
that this may well be a better way to get power users to
continue experimenting with VE (rather than disabling it in
the preference) has significant merit. It could be deprecated
once VE has built-in source mode switching.