To be honest, I don't understand, why the Instrument_name_engraver is not
added to all staff group contexts by default. It is a very simple engraver, so
it would not induce much overhead. And it does not have any side effects. On
the other hand, it happens regularly that you want a staff group to display an
instrument name, too.
I don't see any argument against adding the engraver also to all staff groups.
(I'm already doing that in my orchestrallily package, so I can tell from my
experience that I'm not running into any problem).
From a user's point of view, if he sets an instrument name, he wants it to be
shown, and if he doesn't set an instrument name, the engraver will not do
anything, anyway.
So, from that POV, the sane thing would be to add it to staff groups, too.