It should be noted that this is only the case for Guild Wars 2, and the complete and total opposite was true for Guild Wars 1. GW1, with the whole secondary profession thing and necs being the only ones to have access to soul reaping(for anyone who didn't play gw1, soul reaping was pretty much constant energy (mana) as long as things died, which they did) were able to play any* role desirable for pve.
*Necs "tanked" through the use of minions, and were actually quite squishy on their own in GW1.
In GW2, necs seem to be the opposite, and I blame the extremely simplified combat system. In GW1, necs could support like you wouldn't believe. A blood nec could put the entire team on steroids; feeding energy and brute force to everyone else at the cost of their own health, which they could easily regain so long as they weren't being nailed by big hits. A death nec could swarm the enemy with loads of minions, which when they died, would refund some of the energy it cost to make them(via soul reaping), forming a strong front-line and making it impossible for melee AI to reach your backline. Curses necs were more for pvp, and their focus was around making everyone miserable. If good curse nec was on the enemy team, you wouldn't be allowed to heal, or hit, or protect yourself, and what's worse is there were even curses spells that increased the damage on a specific target. Oh, the glory days of the necromancer.
The problem is that all of this was done through specific spells with their own specific effects. Necs could apply and control conditions, but they were also dependent on hexes(similar to conditions, but each one had its own specific effect, instead of this overly simplified system we have now). Blood magic operated mostly through enchantments(opposite of hexes, so they were like boons but with their own specific effects) and lifesteal being its own "damage type" rather than just gaining health while dealing damage. Even just the small details like the removal of damage types makes a huge impact on professions that profited off of such things.
Necromancers annihilated defenses, slowed physical attacks to a snail's pace, and allowed their own team to hit harder and more often.
Another large issue is just that GW1 pve actually had enemies who applied conditions, put up their own defenses, and needed to be responded to with some form of thought and tactics, rather than just "Kill it ASAP".
Now with that in mind, how the hell do you implement that sort of "flavour" into a game with no hexes or enchantments? How can I have my five different curses spells- each one for a different situation or enemy -when all of the skills in the game are just different versions of the same thing?
Mesmers suffered the same deal with losing all of their hexes and real interrupts, but their solution was to simply turn the mind-games spells into bullshit. Bullshit galore. Just bullshit everywhere you look.
I feel like the necromancer of GW2 is a decent attempt at keeping the spirit of the GW1 necromancer in a game where it just doesn't belong. The necromancer of GW2 cannot compare to the necromancer of GW1. Don't get me wrong, I still love the necromancer profession, even if only for the idea of being the nasty-sicko-anti-hero, but it just isn't the same as it used to be.