* Bone Armor (1+): Absorbs physical damage. Good for keeping you alive when up against melee monsters.
The synergy from wall/prison is better than points into bone armor iirc. +15 points/level from synergy compared to +10 points/level into bone armor.
It is mentioned. It is up to you whether or not you want to place more points into Bone Armor, hence the "1+." However, thank you for mentioning that Bone Wall/Prison gives +15 rather than +10.* Bone Wall (1+): I chose to max this skill and use Marrowwalks for the synergy bonus. With this, I get a 930 damage absorb Bone Armor. It is also useful for keeping you safe from groups of monsters. I find it handy in Baal Runs. Cast it in front of you, and spam Corpse Explosion for easy kills.
* Bone Prison (1 or None): 1 point if you do not plan on using Marrowwalks. Like Bone Wall, it is also useful for keeping you safe but will trap the enemy in a ring of bone instead of creating a wall.