why do glue records *always* *have* to overwrite the cache ?

[Gabriel Somlo]

Simple question, like the subject says. If I have www.foo.com in my
cache as 1.2.3.4, and I get a (faked) response saying "to find
out abc123.foo.com go ask www.foo.com at 5.6.7.8", why do I
*absolutely* *have to* overwrite what I already know (1.2.3.4) with
what's in the glue record (5.6.7.8) ?

If the cached entry conflicts with the contents of the glue record,
there's obviously something fishy, and even rfc 2181 says I should
not overwrite my cache if its contents came from an authoritative
server (section 5.4.1 of the RFC) ?

Everyone seems to take it for granted that "glue records must overwrite
the cache", even bind-9.5.1b1/lib/dns/resolver.c, line 4106 explicitly
forces the glue data to take precedence over existing cache contents.

Please help me understand why this has to be that way ?

Thanks,
Gabriel