Isn't the cohort because they were called 'skink cohort' in the Red Crest army? From the Storm of Chaos campaign?
It also helps that when you talk about skinks, you can devide them into 2: skink skirmishers and skink cohort. Or in short: skirmishers, and cohorts. Easier for regular talk.
The use:
Redirector, surprise charge, and don't get mad: war machine hunt.
Redirecting with cohorts is just so much easier than with skirmishers. Our skirmishing friends have to be set up quite precise, and is fiddly when they're charged.
With a cohort, just plonk it infront; turn it around a bit; And you're set! And they're the cheapest redirector we've got. And they do it quite well.
The Surprise charge: see above. My fellow board-members handled that part quite well. Don't forget that a flank charge on a unit of cavalry is often worth it, for holding them up. make sure your general/BSB (preferably both) is in range. YOu can hold up a unit of cavalry for 1 combat round with pretty good certainty. 2 is also quite likely, but after that; be prepared that you're skinks will die.
Fast cavalry is also a juicy target (if they are stupid enough to show their flank!).
Last but not least: War Machine hunt.
No, they aren't ideal at all for this job. Yes, it does surprise the opponent. Instead of annoying your opponents units, you just march straight past them; straight at the target war machine. If the crew also holds a mage; great!
This doesn't work on: Reapers/Repeater bolt throwers (they can just mow the skinks away) and the Organ gun.
BUT, if you present more than 1 target; they cannot shoot them all and they will get charged. For instance, your 10 skink cohort rushed up, your terradons are in position And your chameleon skinks broke cover to make the charge. Game on!
Don't forget the javelins! They will do nothing if you expect them to make a kill, but they will suddenly kill a random blood knight for their troubles!

(or something like that)
You can fire all ranks of javelins on a large target (read: Giant, HPA, Greater daemon)
In summary:
Yes, I do find them usefull. Even if it's just for practical reasons (easier to redirect with them, than their skirmishing counterparts).
They also have a surprise effect, as your opponent expects your skirmishers to do most of the dirty work.
They are very cheap.
They look pretty cool on the battlefield.
They make your opponent think.
They make your opponent happy. <-- Everybody loves smashing skinks!
The Hunted
Skrox are a totally different matter, which I will comment on later.
