I just use the normal HB/NS rotation against vehicles too. Plague strike hits slightly harder and the dot is useful, but not worth setting up a separate rotation IMO :P

In regard to the single target rotation for obliterate, I do find that it is a really nice way to land some good burst and avoid breaking cc. Although HB + NS is good for damage, it does take up 2 GCDs and the NS gets a significant amount of "damage" from the absorb. If I have a target at <100% health who has blown a defensive cooldown, I like hitting this single target combo to push for the kill. I will generally get 1 oblit, 1 auto attack and 1 frost strike in the same 2 GCDs with at least 1 of them critting. This is generally twice as much damage as the HB/NS combo for a single target.