Nope. The whole point is to prevent creatures from moving through. There can be a thousand goblins swordsmen swarming the fortification, and all they can do is get pegged with bolts and stuff. A thousand marksgoblins and it's a little less one-sided.
That's pretty lame then. So essentially the game has three redundant structures with this function: fortification, vertical bars and wall grates. You would thing that fortifications at least stand a chance at blocking projectiles - hence the name fortification.
I believe fortifications have a /chance/ at blocking projectiles, but it's just that - a chance. In 40d, you could certainly see the piles of arrows in the channels below my fortifications, and watch the hail of incoming fire simply /stop/ at the fortifications with none getting into the room beyond. But there were always 'lucky shot' issues, and Elite Snipers were a real problem, since they seemed much better at getting arrows through the fortifications - and a dwarven archer that got hit generally seemed to die, either from that lone wound or by repeat hits from the machinegun-esque Elite Archer who got the first arrow through.
When I experimented with using grates as removable, permiable walls for my entrance maze, they seemed to have some of the arrow-blocking features, but not nearly to the extent of fortifications. They mostly blocked shots from the 'top', ie bolts that wanted to descend through that square, but instead 'landed' on the top of the grate. Bolts and arrows trying to move through the same z-level as the grate seemed to have very little issue, and that experience cemented in my mind that fortifications, while imperfect, were certainly treated differently than grates. I've never experimented much with vertical bars, so I don't actually know about their behavior, though I suspect they'll be like grates.
That was all 40d. With archers having been nonfunctional for most of 31.x's lifespan, I haven't had a chance to mess with fortifications much in this version - though I will agree with the OP's comment about arrows being attracted to spines. I think over half of my dwarven-fighter-death-by-arrow cases have included an arrow hit to the spine (usually the first hit they took, too).