[Exim] $local_part_suffix outside of routers

Is there any particular reason why $local_part_suffix (and I guess
$local_part_prefix too) is not accessible from ACLs (I have checked
with a rcpt one)?

Well, I guess I know, you cannot set local_part_suffix_optional and
local_part_suffix outside of a router, so my question becomes, why
cannot these option be set globally (and perhaps be overridded by the
router one?)?

I ask as I see that $local_part_suffix is acceptable in an ACL, only
that it expands, naturally, to the empty string:

warn    log_message = X-local_part $local_part_suffix

gives X-local_part

Of course there's a workaround, as I can match $local_part against suffix$.

Why do I want it? Because the $local_part_suffix is not only required
to check existence of a local user, it can also be used to
conditionally accept recipients, for example.

Sorry if the question is too stupid, but I should perhaps take some sleep.

Thanks

Giuliano