Globally apply xref-etags-mode

[Matt]

I'm working from a local copy of a specific org-mode checkout and want xref to 
navigate within the checkout directory, not within the currently loaded Org 
mode (which is not /tmp/org-mode).

It seems like xref-etags-mode is what I need.  However, it only appears to 
apply locally.  I cannot get it to function globally; I must call it on every 
buffer.

For example, I've built and visited TAGS for /tmp/org-mode.  

cd /tmp
git clone https://git.savannah.gnu.org/git/emacs/org-mode.git
cd org-mode
git -c advice.detachedHead=false checkout 
9183e3c723b812360d1042196416d521db590e9f
ctags -eR .

(find-file "/tmp/org-mode/lisp/ob-shell.el")
(visit-tags-table "/tmp/org-mode/TAGS")

I call M-x xref-etags-mode in the buffer visiting ob-shell.el.  I search for 
"babel-eval" and call xref-find-definitions.  This takes me to 
/tmp/org-mode/lisp/ob-eval.el as expected.  However, if I check C-h v 
xref-etags-mode, I see that it's nil in the ob-eval.el buffer.  Calling 
xref-find-definitions on "org-assert-version", it fails to take me anywhere.  
(When I run it with my full init, it takes me to the Emacs system directory 
lisp/org/org-macs.el.gz.)  Calling M-x xref-etags-mode within the buffer 
visiting ob-eval.el allows me to call xref-find-definitions and takes me to the 
expected file, /tmp/org-mode/lisp/org-macs.el.

I can set a hook that calls xref-etags-mode after every jump:

   (add-hook 'xref-after-jump-hook (lambda () (xref-etags-mode 1)))

However, I find all this behavior unexpected.  I would expect xref-etags-mode 
to apply globally.  It's not clear to me why it's only buffer local.  If 
there's some reason I'm missing, then I would expect to see a 
global-xref-etags-mode function.  This don't seem to exist.

Is this expected behavior?  If not, should I put in a PR that adds :global t to 
the define-minor-mode xref-etags-mode?