Poison Skill's Base damage is approximately 22% of Magic attack Plus skill Base.
Dark Skills range from 20-40% of Magic attack plus skill base.
 

Magic attack also adds to poison ticks:
1 poison per 50 magic attack up to 400 magic attack;
1.5 poison per 50 magic up to 800 magic attack;
2 poison per 50 801+ magic attack.
 

Here some issues...
584 Magic Attack, by your logic 400 + 184 = +8*1 + 3~3.5*1.5 = 12.5~13.25

Poison Arrow level 3 : damage 35~40
Real damage: 48 per second (100% the same)
By Your logic: 12.5 ~ 13.25 + 35~40 = 47.5 ~ 53.25
Ok. Minimum damage could be solution.

Main problem with Poison Cloud level 3: 30 ~ 45
Real damage: 66 per second (stable - 100% the same)
By Your logic: 12.5 ~ 13.25 + 30 ~ 45 = 42.5 ~ 58.25... does not match 66...

Thanks.

With Best Regards, Fireball  

Actually Phoenix is wrong.

One of a Staff's Attributes that enchanting enhances IS magic attack, at a rate of 40 per enchant level (+400 at level 10). Enchanting ALSO enhances staffs by 10 physical attack, so they double as a viable melee weapon.

As for the purposes:

Generally speaking,

Heals are 66% of Magic attack + the heal skill's base.
Poison Skill's Base damage is approximately 22% of Magic attack Plus skill Base.
Dark Skills range from 20-40% of Magic attack plus skill base.

Magic attack also adds to poison ticks:
1 poison per 50 magic attack up to 400 magic attack;
1.5 poison per 50 magic up to 800 magic attack;
2 poison per 50 801+ magic attack.

In other words; yes your staff needs enchanted, or you will not be a potent damage dealer, healer.

(Staff enchant can easily be credited with 30% of a SM's total damage potential).

Personally I think the wiki is lacking in regards to information at all on enchanting atm. It says nothing in regards to rates, power stones, protection scrolls, cost to enchant, possibility of enchanting failer costs. It's just vague flavor text at this time.

Word of advice to god:

While rares are pretty inexpensive to enchant, Heroics have a fee that start at 10,000 gold and raise 1000 a level; Heroics have a fee that starts at 30,000 and raises 5000 a level; Legendaries have a fee that starts at 100,000 per try, and I haven't personally experimented with them to see the increment in increases (I wouldn't doubt 10,000 per level)

Power Stones:
I don't think anyone knows quite how they work now. In CB, they had % increases to chance of success that increased 8% per level of stone. Now, they read as follows:

Power Stone 1.1:
Helps Uncommon (Green or lower) up to 4
Power Stone 1.2:
Helps Uncommon (Green) up to 4 with greater chance
Power Stone 2.1:
Helps Rare (Blue) up to 6
2.2:
Blue up to 6 greater chances
3.1:
Up to Epic, 9
4:1/2
Up to Heroic, 12
5.1:
Up to Heroic, 14

And I'm guessing 6.1/2 is Up to Legendary, to 15.

(In other words, if the new flavor text is true, that means be careful what stones you use, although it's pretty easy to get [5]-[6] just on gold alone).  

...
What I said was in reference to the tick damage; the numbers you are listing are derived from the skill base additions (the skills do not describe how much they actually add for tick damage).
 

I'm pretty sure Poison Arrow's Base tick is 32, and Poison Cloud's Base tick is 54 (at lv3, they do 22 points difference in damage).