The Necromancer is Underpowered Reason 6: Curses sure are useful (to paladins)

Every class gets three evenly distributed skill trees, but not all skill trees are equal. The paladin has a tree full of defensive auras, but he can only have one of his auras active at a time and the offensive ones are so much better that there's no point investing heavily in defensive auras except as synergies. I'd say that the assassin is an example of well balanced skill trees: just about any assassin can invest in all three trees. Some individual skills are lackluster, but the trees themselves work well together. And I'd say that the druid is an example of poorly balanced skill trees: shapeshifting precludes elemental spells, so it's either one or the other, which forces player to build from a very limited set of archetypes.

I've complained enough about the necromancer's poison and bone skill tree (investing in either poison or bone only allows for one type of damage and takes up too many points to have another primary damage source) and about the summoning tree (skeletons are unreliable). That leaves curses. There is no curse that deals damage to enemies (even though there totally should be), so they can't be the central focus of any build (that actually works). They are designed for utility. The tree as a whole does have its problems. Some monsters cannot be cursed. Because curses are targeted on enemies, rather than passively affecting one's own party, they're clumsier than auras. But more annoyingly, most of the good curses don't actually get better with multiple points invested in them. The magnitude of the effect generally stays the same, and more points into the skill only improve the duration.

Four of the curses affect monster behavior. These crowd control curses mostly only affect weak monsters, so they're not worthy of much consideration. I usually leave them alone. Of the remaining six curses, two more of them are bad. By this, I mean they are not good curses. They suck. Iron Maiden is unusable outside of Normal and Weaken is garbage. That leaves four curses that are actually worth having: Amplify Damage, Decrepify, Life Tap, and Lower Resist. Of these four, three are one-point wonders. Putting more points into these curses only improves the affected radius (easily dealt with by spamming the curse over a wide area) and the duration (easily dealt with by spamming the curse). Lower Resist does get a magnitude increase with greater investment, but unlike the paladin's Conviction, it has severely diminishing returns.

All four of the good curses would be great for a necromancer playing in a party, supporting his allies by crippling monsters for easy kills. But for a solo necromancer, curses are harder to take advantage of. For another class, items grant curses, usually by having a chance to cast them on striking, allow these curses to shine much moreso than they do in the hands of a necromancer. Amplify Damage may complement Corpse Explosion, but it's so much deadlier when a barbarian is using it. Runewords and unique gear let the other classes put Amplify Damage, Decrepify, and Life Tap to better use than necromancers can. And a sorceress or other elemental damage-dealer would wreak havoc with Lower Resist if there were a decent item for it. On the necromancer, if not for breaking poison immunities, it would be entirely wasted. Life Tap is amazing, but it only synergizes with high physical melee damage, something necromancers don't have. Decrepify is similar to Amplify Damage in this regard, but the damage reduction and slow components benefit a melee character more than one that sits back casting spells or hides behind a wall of pets. So yeah, the necromancer gets some good skills with these four curses, but the best uses of these skills are all demonstrated by non-necromancers.

The strange thing about curses is that it's almost as though Blizzard is doing this part of the whole "make necromancers the weakest" thing on purpose. Items that grant auras are some of the best in the game, but they're never made so strong that they outcompete the paladin's own ability to use auras. Other characters can get auras, but they can't get the highest level auras and they can't get the synergies that auras get from other paladin skills. Curses don't improve much with level and don't have any synergies for some reason (synergies were not part of the original game, but were added retroactively, and curses just never got any for some reason). The end result is that getting curses from items while having access to strong weapon-based damage skills is a much better combination than having those curses natively, but lacking skills to go along with them. Lower Resist is the lone exception only because it's the one good curse that gets better with more points and the two items that do give other classes access to Lower Resist are crap.

This is the area where I see the most potential room for improvement. Curses can be buffed considerably without making the necromancer broken. Weaken should be cut and replaced with something that doesn't suck. There should be a curse that helps the bone skills. Lower Resist should lose its diminishing returns. Iron Maiden could use a rework (Blood Golem finally got one, but Iron Maiden is still outdated). Amplify Damage, Decrepify, and Life Tap could be changed so that they gain some benefits from having points added to them other than range and duration. Finally, the curses should have synergies, just like most skills do now.