Let's look at another creature that might be teamed up with it.
Is 18 damage, plus healing 15, plus taking away a healing surge, plus making a target weakened severe enough? That's a 33 point swing, plus 1/4th of your HP due to the loss of a healing surge. It kills a fighter with high con in about 9 hits, and together with the Death Hag, cripples you for later fights, possibly killing you straight out. With no healing surges, the cleric can't use Healing Word, you can't recover with a natural 20 on a death saving throw, and so on.

Okay first, death hag's Claw attack marks you, making your attacking others harder (as you take a penalty and provoke an AoO. Second, Life Drain is a Burst attack, meaning it affects everying in a 9 square area, with a high attack.

Likely to hit everyone, but possibly not so...9 damage per person in 9 squares, for a 5 member party that's 9-45 damage and heals the hag from 5-25 hit points. Plus it's got the possibility of sealing healing surges.

You'll fight maybe 2 of those, along with a handful of minions and an artillary.

They'd be helpful in combay.

You can't look at one monster and go "it sucks", unless it's a solo. The hag, in its role, is a frightening, incredibly dangerous monster, as the others have pointed out. It doesn't deal massive damage, because that's not its job - it keeps you busy. And it does that brilliantly. If the hag does its job, you have two choices: Ignore it and take ~10 damage a round, and have your main attacker suffer -2 to attack; or, attack it, and spend round after round hacking it apart, getting blasted by artillery monsters.

Consider a pair of death hags and a couple of night hags, just a few hundred xp above a level 16 encounter for four PCs. The death hags are attacking your fighter and, say, rogue. They can't heal, and even an average of 11.5 damage a hit will wear away - a level 16 fighter with 15 Con has 120 hp.

Meanwhile, the night hags open up with a blast that deals ~7 damage, and party members may be taken out of the fight. Then, they deal an average of 14 damage, a maximum of 22. You may be conditioned to 3.5, but in 4e, 15-20 damage hurts. Consider that that's 1/8th to 1/6th of the fighter's hp, gone. Should he hit the wizard, he could be down around a quarter of his hp each round.

So, yeah, the death hag, by itself, is useless. If you fight it by itself, your DM's stuffed up.

Indeed, I'm running a 8 man game... does that mean 8 monsters... no.

Often it will be more

It depends how you DM, if you say do a large group of monster with the same init all at once that cuts time in like half...

It just bugs me that there has to be so much team thinking, you know?
A level 18 soldier shouldn't be something "worthless" unless it has several thousand XP and several hundred HP of back up bad guys that synergizes just so with it.

20 person combats?
Jeesh.
I feel bad.

That's the major change from 3.5 to 4e. 4e is all about working in teams, both the PC's and the Monsters. So, it would seem, that if your going to move forward with 4e, that you would need to start embracing the "team think".

It's realism intruding into the game. Any sort of army is all about working as a team. And since a lot of monsters are at least as smart as humans, they embrace the same tactic.

Even wolves work together as a pack.

Alright so, I just ran my first campaign....encounter sizes?

8-12 monsters in each one.

Combat took...30 minutes to an hour each time. Less than a normal round.