Bone is 100% useful vs physical immunes and useless on anything else. Bone break puts them at 95% phys immune, amplify damage makes it -5%. And don't listen to noobs who'll tell you amplify damage and decrepify work at 1/5 only after sunder without testing. They do so with no sunder present.
But does it affect the skeleton attacks? Would it be worth it for a summoner to get one nat the prices theyre at atm on ladder?