Really the other two specs are shit because you cannot get away from Deathsight.
The base lifetap is your only 1500 ranged DD and sub-50 sight introduces variance into its damage. No other line offers one
The ABS buff from sight is the only absolute means of melee absorption for your pet. Straight painworking STR/CON is in the negative vs str/con or con debuff, while straight servant DEX/QUI is in the negative vs dex/qui or dex debuff.
Without powertap your powerbar is fucked. Without 44 sight you're not going to want to use it in combat. With 34 sight you can (most of the time) rely on it in a pinch to pull some power. Anything below that is impractical. Notably the servant spec shadenuke has big power costs
Servants's lack of utility makes it difficult to spec high into, leaving the only spammable shadespell as the shadetap from sight. The shadetap at 39 sight is passable. While Servant offers a shadenuke at 38 the line offers -l-i-t-t-l-e- shitty bulk and no means of damage beyond 500 range.
For painworking it's pointless to spam the shadedot but high painworking can't go far into one or the other for a shadedd.
Because the unbreakable snare is aoe the effect on most targets more often than not goes wasted
The real value in painworking is the buff being the only defense for necro against magic damage, but on its own not worth losing the ABS from sight. Yellow ABS with yellow str/con almost compensates for the loss of red ABS when combined with the str/con debuff, but only if it's active prior to taking hits
You could boost the pbaoe delve to 400 and servant spec would still be shit in rvr. Putting aside other issues, the tether on that spell gimps it hard. Painworking is essentially the same thing (pet pbaoe & a 500 shadecast) but pain's pbaoe doesn't suffer from falloff because it's a dot and you don't have to be on top of the pet to get the pet to cast it, and although the dot invariably has 500 range it too can be casted from a distance. This permits the necro to hit moving targets so long as the pet is on them, whereas servant can't do anything if the target is not stationary. Sight falls in between where the shade can't do shit to a moving target but the pet can nuke them down (lifetap,powertap) . Using the base snare still will not allow a servant necro to use his casts on targets bent upon not hanging around
Provided cleric buffs any necro becomes a tank however, yet sight once again is the only spec providing more (ABS buff)