K, so just with some quick napkin math, this is quite simple. Using this log (yes I realize it's pve, but it's irrelevant to illustrate this point)
http://www.worldoflogs.com/reports/r...?s=1291&e=1519
We can see that the avg crit of frost strike (this is dw so combine both mh and oh) you have 57,211. avg crit of OB is 65,873. An 8k Difference. Even IF you add in the actual absorption amount of necrotic strike (this is with 2h, so dw would be even lower) it is about 9k, plus a howling blast that (according to this log) crits for 54k.
SO in order for necro plus hb to be better than OB, you would have to be 2H, plus you would have to get a HB crit to even come close. I know you are going to say *lol armor mitigation nub blah blah* BUT why are you even using hb w/o a rime proc in the first place? (unless of course they are kiting you) With Rime, you get FREE HB and are then able to use the needed runes on OB instead. Not to mention that thanks to Killing machine, you can GUARANTEE those OB crits, rather than the incredibly low crit chance of HB due to reforging everything out of crit.
It actually isn't hard to get this string pulled off at all. Whats the worst class to pull it off on? A mage? AMS prevents any sort of snare, poly, or whatever else they want t throw out at you, plus death grip if they try and get away, and a ranged, 5 second silence preventing them from trying to blink AND your own snare via chillblains. Not to mention the fun you get to have if you catch a Ring of frost, or mirror image with dark sim on them. So go ahead and waste runes on your necro strike and weak HB. I'm going to be killing clothies within 10-15 seconds, who have 4.5k resilience, while my partner CC's the clothies support.
Edit: forgot to mention merciless combat, which increases OB damage EVEN FURTHER when the target is 35% hp or less