[exim-dev] Expanded options vs non-expanded

Today, with so many options expanded, and so much flexibility, does it
still make sense to have non-expanded options?

I know that the non-expanded options can be checked at -bt time, but so
much is expanded that this buys you fairly little.

How do people feel about making all options subject to expansion? And
if there is no '$' in the value, then the item gets checked for syntax
immediately?

Or perhaps "only booleans which are somehow intrinsic to the meaning are
not expanded"?

Is there something I'm missing?

Another option, if we want to make items be syntax-checked by default,
would be a new *= operator for assignment, or $=. But that just leads
to historical quirks and I don't much like the approach.

-Phil