I'm personally very happy that some of the rule changes have made the Necromancer a viable model. I enjoyed playing it for thematic purposes in the past but for the points it just never quite felt like the best choice. Khamul (also very thematic) plus a high number of troops could be brought along for the same price and do a better overall job. The Necromancer was just never enough of a threat and was actually a liability.
Would you consider him useful below 750, such as a 600pt game? My gut instinct would say no but we haven't adopted the Hobbit rules locally yet so I'm still looking at things thru 'classic' SBG eyes.
As for Casteallans, they are tough and can do a good job in combat but they are also not usually best for the points...in 'classic' SBG. I would use one sometimes in a 500+ point list but not always. With no Might and no ability to call Stand Fast they just lacked the flexibility of a Hero but at Hero points. With the changes they are more reasonably pointed and so including one in a 500 point list looks better. I don't think I'd get two though unless I was going with a lot of affordable basic troops. In either case....spend the extra few points and get the Morgul Blade.
). If he wasn't surrounded and seriously outnumbered he might have lasted even longer and take a few with him, but even with double strikes it still took them two turns to burn thru all his Will (Fate). I have always found them to be hard models to kill off if played well and the Morgul Blade is always a good scare tactic. Often an opponent may avoid putting a major Hero against him "just in case".