Bottom line to me is that Shinkurou knew the Kuhouins were likely to attack in the first place; the second he got to the apartment and saw what was going on, he should've known what kind of opponent he was up against and decided to use the bone if he could.
That being said, at least he didn't yell and raise his power level and magically trump people who were clearly leagues beyond him in both combat skill and numbers. People complain that a simple bone implant being used as a weapon is unrealistic(I guess the fact that it's supposed to be magical/demonic gives credit to that criticism), and I hope it's not those same people complaining that he didn't pop out his blade and magically either get close to winning or beat them.
I think there's credit to the idea that he should've used the bone if he really cared, but it seems it's not a plot device analogous to shouting and raising his power level(we'll have to see if that ever happens). And yes there are drawbacks to it, it not only leaves him open while he forces it out, and it leaves his arm injured/disabled after use, too. I don't think that should've stopped him from using it, though.
edited and reposted because i failed too much in the inital writing