I like what floyd said. When Janice does a bluesy tune I'll sometimes
say "want to do some harmony?" and her polite response is invariably
that the style doesn't need it. Janice and I both came from years of
bluegrass where EVERY song had harmony most 3rd's and 5th's above
and below, etc., so it was an adjustment not to just throw in harmony
for no reason other than she can do it smile

I completely agree that your tunes are the type that really showcases
the vocal and, in most cases, no matter how well done harmony would
not be needed. FWIW, I ascribe to the notion that "perfection is not
when there's nothing left to add but rather when there is nothing left
to take away.

J&B