Tobab - I think you were onto something but dismissed it a little too quickly.
Your model is a little off. Okay you have 20 mandrakes that only kill 4 Marines. But your not going to lose as many as you say in return...why? Well for one i've never seen a squad of 20 Marines, they come in packs of 10. So really your math should look like this. 10 Marines get 10 attacks, 5 hits, 3-4 wound, and 2-3 Kill. Well dang, the Mandrakes just won combat. Meaning the space marines could easily break (17 mandrakes vs. 7 Marines - that's 2 to 1 and you may get lucky and get 3 to 1) and if we pursue, combat gets moved back another 2-12" (moving space marines backwards is always good). At the least you'd tie up a very nasty Marine unit for a good turn or two.
And it'd be even more beneficial if it was a devestator squad you were going against.
dang, maybe i'll proxy some of these guys to see if this'll work. I've just decieded vs. fielding my grotesques w/ Haemonculi to shield the Talos. Now i'm using the Talos to protect the Warrior mobs, so i just freed up around 200 pts......... I think this idea has merit