You're totally right about the way it works (except for your example; the generated macro actually has (defvar-impl ,'foo) which is effectively the same), but being able to choose isn't the issue, I think.
If quasiquote didn't account for nesting, all we'd have to do is this:
To clarify a bit, the ,', idiom doesn't have the same effect in both cases. It essentially breaks out of nested quasiquotes and breaks into non-nested ones. This is thanks to the fact that in the nested case the inner unquote applies first, and in the non-nested case the outer unquote applies first.
We can even simulate this non-nested behavior right now by writing our quasiquotes within ssyntax, so as to hide the inner ones from the quasiquote compiler:
(And for what it's worth, this change should affect the generated macro by removing the extraneous ,' in (defvar-impl ,'foo).)
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With this in mind, why do we use nested quasiquotes? I'm guessing it's because code is easier to edit this way. We get to wrap a quasiquote template in another template (or to reverse that process) without having to go through and edit every single unquote form. We only edit the unquotes that break out of the wrapping, and we'd have to edit those anyway.
