After years (literally) of using a southlands all skink army, I am inclined to opt for skirmishers every time.
I find the cohorts tend to... how shall I put it... get in the way and do no real damage where I need it to be done. It is probably due to the fact that I am still getting to grips with manouvering rank and file troops but I can never seem to find any real use for the little guys in a ranked formation.
The skirmishers on the other hand... ahhh. Lizards of the match every time! I am trying to keep my skirmishing units to a minimum (2 or 3 units) and they always tend to perform rather well.
I find skirmishers useful for a number of tasks. First off, their speed and freedom of movement ensures that I can get them where I want them when I want them there. Granted that in the 7th edition skirmishers may no longer "run" if within 8" of the enemy (which was a great blow to my all skirmishing army) but they are still very useful... you just need to be more careful.
Secondly, they are great at protecting flanks and rears from fast moving nasties such as fast cavalry. This can be carried into effect in a number of ways. The most obvious is that if positioned correctly, you may block your more valuable troops from charges and position the skinks in such a way as to redirect them to your liking when/if you flee. They are also great at taking out fast cavalry with poisoned darts, as well as being a threat to such things as the dwarf gyrocopter (which you can charge and keep tied down for the rest of the game).
Thirdly, they block the opponents march moves. I find this to be of great value especially in the 3rd/4th turns when troops start taking positions for the charge. This, together with reason "one" above make skirmishers my most useful and versatile units in the game.
Another good use is the sniping of giants and other low armour big guys

. My skinks have amassed a whole collection of giant,minotaur and ogre heads over the years.
Finally, if after performing in all/some of the above roles, 5 skinks in a unit are still alive at the end of turn 5, they can safely run towards uncontested quarters for those sweet extra VPs.