Staff is excellent for crowd control and party support, and it's overall a great weapon.
Double Daggers spikes really hard, burns/ bleeds really hard, and it's a very fun, high-risk, high-reward mobile weapon. It's also full of AoE attacks, like the Staff, so you can take credit for multiple kills at events, but it has far less combo fields.
Scepter is underpowered, and Focus even more so. They're more defensive and safe than daggers, but Scepter is too focused on single target damage (yet, it's somewhat weaker than dagger), and Focus skills are mostly underpowered (both fire skills) or highly conditional. Earth Focus is quite strong, though, and Scepter + Focus have the best condition removal of all weapon sets.
Best Staff (excellent with many, good for solo), Double Daggers (excellent for solo, good for many) Scepter + OH Dagger (good for any situation) Dagger + Focus (crappy fire focus skills) Scepter + Focus Weakest
Scepter is underpowered, and Focus even more so. They're more defensive and safe than daggers, but Scepter is too focused on single target damage (yet, it's somewhat weaker than dagger)
It's a bit weaker because you can stay at range with it, so you're safer and can kite a bit. With daggers, you basically have to be really close to the enemies, that's why there's a higher risk to damage tradeoff.
On that note, Staff > All!
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Scepter/Focus have exceptionally limited use, it mostly comes down to staff vs. daggers.
Staff is good for soloing, generally. It is also the best burst healing with water, layering rain and geyser on top of each other. However, you have a sizeable wait in between heals when you do that, and your utility between heals is limited to spamming ice spike to stack vulnerability -- hope the mobs aren't moving much.
Dagger is good for partying, where enemy focus can be shifted away from the elementalist. Daggers are high risk, high reward weapons, and when partnered with other professions elementalists can afford to play in the in-close game that daggers require. Water daggers is exceptionally good for duoing with a melee type like a warrior; your boomerang spam stacks up vulnerability (on multiple targets if you position yourself correctly), which increases the warrior's damage considerably, and water breath (I don't remember skill names, just what they do) is nice spike damage while also healing your partner if you stand behind them; it also comes up fairly often. You also get a condition remove with the fifth skill, that big water splash, which you don't get with the staff.
I played a lot of the elementalist class during all of the BWEs and during them, almost every elementalist used scepter/dagger or staff.
Personally, I've always wanted to use dagger/dagger. But during the BWEs, you kind of shot yourself in the foot if you did. My only complain during the BWEs was that the dagger/dagger damage was too low, compared to being forced to be in melee.
They fixed that.
Dagger/Dagger does more damage then staff. But staff has range. Specc traits and items defensively and dagger/dagger is comparable all the way. (Staff users have Rain of Fire, dagger has Churning Earth)
Focus is kind of lackluster, I agree.. But I think it has its uses depending on how you specc.
Scepter/Dagger is still a favorite for pvp though for many. Since they have good synergy. (Ring of Fire -> Dragon's Tooth -> Phoenix -> Flame Grab = One of the best bursts in the game. Neither daggers or staves can compare.) This has its uses in pve too though.