So fun fact: I actually changed my own opinion within writing this post.
Bladelock, I am in the for camp, and I think shadow weapon does not get the bonus for the same reason Summon Monster doesn't get the bonus: The spell description has no Dice-based damage roll to which to add the bonus.
THAT SAID, that spell, like all the other spells that manufacture weapons, have their own internal limiters on damage, plus you have a spell cast making attack rolls each turn. +1-2 damage on a hit is not a huge issue. In fact, its also very flavorful for an Orc spellcaster to just summon up a weapon and charge into battle. So if I was GM'ing and my player asked, I would give the minor damage boost.
Rules though, the spell description does not have a damage roll in it, so it would not get the bonus damage. The line is unclear, but how I personally read it, that seems to be a place that makes sense.