Taking a character with the Grimoire and Kairos to let you re roll it. This will give your Screamers a 3+ Invun and you re roll 1's so you essentially have a 2+ Invun save, couple that with 2 wounds each and being jetbikes should mean you get 2 rounds shooting at them max.
Do screamers really have a 4+ invln? I dont have my book with me ATM but I am about 80% sure they just have a 5+ invln and Kairos is the only thing with a 4+ invln. I wont be complaining if they do wind up having such a good save though >_>
+Edit+ I just checked my Daemons Codex and the FAQ and Screamers do indeed just have a 5+ invln save just like all the other daemons (save kairos) +Edit+
I love Screamers for 3 reasons; Cost Efficiency, Flexibility and Speed.
*Screamers are a two wound model with an invln save, decent states, great rules and you can get four of them for just 100 points.
*Screamers are one of the few options in the game that can be used against virtually any target. Their fly-by attacks can mess with light infantry while their lamprey bite can demolish vehicles and even heavy infantry if they manage to get the charge (and being jetbikes that is easy to do). This means that no matter what list you are facing, screamers will always be a good option.
*What was once the providence of only Eldar (Emo or otherwise), being a Jetbike gives Screamers, and the armies they can ally with, some incredible speed that isnt really matched by anything else in the game. This means you can apply your swiss-army-scalpel exactly where you need it when you need it.
When I take Daemon allies with my Thousand Sons this is what I usually wind up taking;
LVL 1 Herald of Tzeentch with Exalted Locus
20 Horrors of Tzeentch
6 Screamers of Tzeentch
= 400 points
Everything in this little detachment can plug a hole in my list and all for a nice, low 400 points;
*The Herald functions as a divination battery providing me with re-rolls. These usually go with the Horror unit, but with them pumping out so much Str 6 shots a turn I feel this is justified.
*The horrors provide a cheap troop unit that can hold an objective and still provide ranged support. Since I play a Thousand Sons list, one of my favorite things to do is deploy an objective in the middle of a large open space and away from terrain. My chaos marines and the Horrors have invulnerable saves so cover is only an ally to my opponent, and by forcing them to approach out in the open towards the horrors, I can get a Rhino full of Rubric marines to pull up alongside and catch them in a crossfire.
*The screamers give my list much needed Close Combat punch and also anti-tank capacity that would be lacking otherwise. (I tend to load up on plasma) and can function as a stop-gap tar pit with their inland save and 2 wounds.