If you have a fully synergized bonemancer I don't see any reason not to use all three skills. I save Bone Spirit for last since the plus skills I get out of a +8 bone spear white wand keep it competetive with Spirit for one on one battles, but teeth is invaluable for taking out large groups of low HP enemies and spear does a great job against groups of tougher targets and in corridors.
I've seen a lot of people play really boring bonemancers. All they do is spam BS. They don't even use walls or curses. Cuts the fun and effectiveness in half IMO.