Don't forget that if you are bonemancer, you most likely have a very high-life bonearmor as well (in fact, even my skelemancer has a very effective bone armor, and he has no points in bonewall...). Bonearmor eats all the physical damage, so if you try to not get swarmed you'll not get ANY physical damage at all.
Also, a lot of the attacks that would be blocked are melee range attacks, and its always a good idea to stay away from the bad guys. If you want to play a barbarian, do not select the necromancer on the selection screen.
There's still elemental missile attacks that can be blocked, though, but Gumby, bonewall and curses can help to divert these elsewhere. Besides, even though I vote against blocking, I always vote for high resists.
Lets look at some numbers:
1000 fire damage missile, no resists, no blocking -> 1000 damage
1000 fire damage missile, 75% resists, no blocking -> 250 damage
1000 fire damage missile, no resists, max block -> 250 damage
1000 fire damage missile, 75% resists, max block -> 63 damage
However, assume you need 200 dex to get the max block, so the "no blocking" build has 400 additional life (IIRC necros get 2 life/vit --- I'm playing a no-blocking necro, but I just dump the points in vita without worrying about the actual results): in this example, the no blocking takes 187 additional damage, but since he has 400 more HPs he's still leading.
Of course, if you change that to 10000 fire damage only the max everything build has a chance to survive --- I don't know whether he'll have the 625 hitpoints, but the others would need 2500... but in order to get 10000 firedamage you pretty much have to have a deathwish to start with.
Of course it's somewhat an artificially constructed example, but I tend to get out of harm's way when harm moves in my direction, so I'm not dealing with a constant long-lasting bombardment of blockable attacks --- it's more like "oops, a bunch of archers, retreat and setup diversions".
