The smaller Ratling, trying to vie for attention to make the
Taking a minimum squad of three is a great way to go. They are cheap (cheaper than nearly any other unit in the game) and are fully equipped to do their job. Taking the minimum squad of 3 models only costs the same as two space marines, but these guys shoot as well as marines and have a rending shot that always wounds on a 4+. Increasing the squad size really doesn't help them much, as it only takes points away from more killy things in your army for little benefit.
Every turn they fire at an enemy unit, they are putting on average a single wound on a model (before armor saves). Every three rounds approximately, you'll end up with a single dead marine. Opponent goes "so what...one dead marine, what do I care?"
Well there is a pinning test involved after that marine is killed. While the chances are relatively low of pinning a unit, the consequences of the opponent failing a pinning test for a key unit can often turn a game (how much would you pay for a chance to pin a squad of Long Fangs for even a single turn?). Using this squad in conjunction with a psyker battle squad can make that likelihood of a failed pinning check even greater. The potential to hamper or paralyze a more expensive unit makes this unit effective, compared to their cost
In the old days, Ratlings were referred to as "Charlie."
If an opponent has a character or monstrous creature, you're presenting an asymmetrical threat to that model. What I mean by that is you have an extremely inexpensive unit with the potential to do much more damage to the enemy unit relative to what the enemy model could do to that unit. If the unit scores at least a single unsaved wound on almost any character or monstrous creature, they have done more than 30 points worth of damage to that model. The best the enemy unit can do to them is to kill a 30 point squad, which doesn't achieve much when dealing with 1500 point or greater armies.
Did I also mention that they can be relatively survivable in cover (thanks to "stealth" USR) and infiltrate? This can be a very inexpensive way to deny enemies from being able to infiltrate close to your lines and to make it more difficult for enemy units to scout close to the main body of your army for a first turn assault (deff-coptas, valkyrie vet rush, etc) as those models must keep 12" away from any of your models, so if you infiltrate out into the middle of the field, you make it harder for the opponent to get a first turn assault on the unit he wants to hit.
For their extremely cheap cost, you're adding another unit to any opponent's already stretched
Force Org Slot competition is not much of an issue either. Most of the things people use in the codex come from troops, heavy support, and fast attack, leaving at least one elite slot open for this unit to be fielded.
In summary, taking a minimum 3 man squad of ratlings is an excellent cheap filler unit that has the potential to be hoisted off of the field in glorious victory!
"Ratlings, Ratlings, Ratlings..."