I can't say for other classes but when it comes to a Necromancer's off-hand, I love the niches that they fill.
There are trade-offs depending on the weapon you choose and feels really nice and balanced.
This is from a WvW roaming perspective - feel free to share your WvW/PvP/PvE perspectives!
Similar to the Necro Staff, the off-hand Dagger is a great utility weapon.
With a decent range at 900, it is the embodiment of abilities that are both defensive and offensive in nature.
Blind, condi transfer, AOE weakness, bleeding, boon corrupt.
Although lacking in damage and the only OH with no LF generator, its versatility is well-suited for use in the dynamic environment of WvW roaming where you'll encounter various styles of opponents.
Personally my most favorite weapon, followed by Staff.
With a long reach at 1200 range, the off-hand Focus is a weapon that specializes in catching out kiting and escaping opponents.
With its chill application and triple boon removal, it is a strong debuff weapon.
Focus 5 also happens to be our longest range and hardest hitting skill.
The newly reworked Focus 4 is also a nifty and more reliable range LF generator, though a downgrade when it comes to the power spike role that Focus used to fill.
It is changing its identity slightly with this rework and perhaps could now be seen as the niche Single Target Long Range Power Spike + LF generator OH weapon.
Our shortest ranged OH weapon, it is a multi-target close-range combat weapon.
Unlike the Focus which is more Single Target, the Warhorn is a weapon better suited for group fights.
With its coned AOE unblockable daze, it is a great weapon for interrupting multiple opponents at key moments.
The reworked Locust Swarm has given this weapon some nifty sustain leech especially when traited with the Blood Magic Warhorn boosting traitline, and its effects are amplified further when surrounded by foes.
Our best defensive sustain OH weapon, I suspect it can heal as well as a Signet of Locust in a group fight.
Vampiric Necro anyone?
With the introduction of Scourge, we got our very first Condi Spike OH weapon in the form of a Torch.
Unlike the dagger which is more balanced, this is a largely offensive weapon that generally feeds off an aggressive play-style.
Perfect for a in-your-face keyboard-mashing Scourge.
CD -> TC -> Mag -> GOM -> AR -> JQ