As others have pointed out, Necrosis is less useful than it may sound like. It is, by no means, a flat 2% dps boost per point, while 2H weapon spec is very close to that level of returns.
If you're gung-ho about having the ghoul available (and I don't blame you at all), consider speccing deep unholy. Unholy is entirely tank viable, all the way through Wrath endgame. Most of my betatanking was done with slight variations on this spec:
http://talent.mmo-champion.com/?deat...50151023103151
Few points worth clarifying:
-Dirge RP returns are fairly poor. While Butchery may seem an odd choice for RP gen, the constant RP ticks are reliable and flexible and the "on kill" effect goes off far more often than you might think. Ultimately, this or Dirge. Matter of personal preference.
-I see a lot of Unholy builds skip Impurity. Don't. Synergizes especially well with Bladed Armor.
-I see a lot of Unholy builds skip the Gargoyle. Don't. It's one of the best burst abilities a DK can get.
-Reaping is less desirable than it may seem. It's handy, but Unholy generally uses its blood runes AS blood runes.