i.e., you could keep the adapter in, and just unplug the antenna cable,

That's exactly what I'd be doing Linus. BTW, my batteries just went out while
typing this, popped in a fresh pair of rechargables (really come in handy and
have saved me lots of money I'm sure.)

but if
you leave the adapter in, the radio will see the adapter as the antenna, and
ignore the whip.

That's fine by me. While the 'adapter' would be plugged in, I wouldn't be
using it. I'm not concerned in the least of it affecting the rod antenna.

I'd say that you should just unplug it when you need to,
and keep a few extra jacks on hand, to replace onboard in the event of the
stock jack breaking.

I don't like that scenario.:-(

but I just wouldn't want to see
someone buy or make an adapter plug which essentially killed their whip
reception when plugged in WITHOUT the attending external antenna.

But I don't understand why you feel that way, I mean I do I suppose. But the
way in that I'm using my external antenna jack by plugging and unplugging,
sooner or later is gonna do damage. Find an 'adapter' to keep plugged into the
external antenna jack all the time, if I'm not listening, then my antenna
won't be plugged into it anyhow. If I leave the house and take it with me,
then I'd take the adapter out of the external jack and use the whip or even for
that matter.....keep the adapter STILL plugged in and unroll my 500ft. spool of
wire that already has the same jack soldered on the end already, that'd plug
right into it anyhow and unroll it (the spool of wire) and DX away. :-)
**Follow me Linus?