The problem we face is an understanding of what constitutes a "rated"
structural member and an appearance grade member. Most of the time in
this group I think that we are refering to relatively small
(1X..)material. In the case of this discussion of birdsmouth spars the
individual staves seem to be 3/4" maximum. I doubt that most 1x
material is structurally rated. Most likely it will be appearance
grade, and the finger joints will not provide anything approaching the
strength of natural timber.
Anyone contemplating the use of finger jointed material should
consider doing their own testing of the joints. Get a length of the
finger jointed material, rip it into strips an inch and a half wide
and break it at the joint and then between the joints. If the break at
the joint is close to 90% of the strength of that between the joints
you can likely be confident of the finger joints strength.