I can confirm that.

Bone spirit even at very high levels has a narrow range of damage.  
Venom lords for example in the hell chaos sanctuary ALWAYS die in 7 hits
(to me, at my current level).  I ran around casting lower resist and
they still all died in 7 hits.  I did the same thing with a couple other
monsters and got the same results.

I also teamed up with a BH pally to see if it helped him.  BH is also
magic damage.  He noticed no change in kill rate when lower resist was
in effect.

The one thing that I find helps with bone spear is decrepify.  Faster
run/walk boots are also a plus.  With no minions and no merc, I
sometimes run around in the open areas in hell (like the black marsh for
example) and all the fast runners follow me.  I zigzag so they stay in a
tight bunch.  When I have the entire world following me, I cast decrep a
few times and start with the bone spear.  You can take out big crowds
this way without needing to cast bone wall/prison.  Of course there are
some areas I won't do this but it sure as hell is fun.

There is also anotgher trick you can try but your minions may screw it
up.  Walking into an unknown area, cast a bone wall far ahead of you.  
Things start attacking it and they are all in a line, but perpendicular
to you.  Cast bone prison around them a few times and walk to the side
of them so they are all in a line and you can max your pierce effect by
hitting many things with one shot.

BS is an "assistant" skill unless you make a serious investment in
synergies.  It's good for going into act 1 and killin some fallens for
your first few corpses.  At level 84 with ALL my points (except 3) in
boe spear and it's synergies, things in act 4 and 5 of hell are a slow
kill.  But it is a safe kill usually so that makes up for it.