Flipping through the entirety of Carl's old Rebol blog, I came across a surprising post in which he considers a trick for implementing DEFAULT using left-quoting in Rebol2.
While it's very odd that there were ways to hack that in, I found myself more curious about the alternative proposal... to have DEFAULT work as a dialect.
x: y: default [10 + 20] ; can't set X
default [x: y: 10 + 20] ; dialect can offer handling of leading SET-WORD!s
This isn't a situation like comparing let x: 10 and let [x: 10]. LET doesn't need the block, so you get something inherently messier. But because the nature of DEFAULT requires the block to suppress evaluation in the case that it doesn't need to do the defaulting, you have it either way. 
>> x: null
>> y: <something>
>> default [x: y: 10 + 20]
== ??? ; by convention, what's the answer?
>> x
== 30
>> y
== <something>
It's a bit of a problem on what the return result would be, and it's a bit misleading because you see on the screen [x: y: 10 + 20] but you get back something where after that x <> y.
I think I'm going to have to go with thinking that the current way is better. But again, I was a bit surprised to see the Rebol2 backwards-quoting-hack.