Venmthrope's rules: any unit within 6" gets the save, doesnt say any unit wholly within 6". There are quite a few examples of this sort of effect. Necron res orb, IG Straken, Ork KFF to name just a few...
Um, well its nice, but not really as good as you may think. 2 reasons why it isnt that good and 1 why it is: venomthropes are really easy to kill (T4 with a 5+ armour/cover save and onyl 2W), gaunt screens give pretty much everything a 4+ cover save... BUT the venomthropes give cover to everything, including monstrous creatures, which is their best use. Throw them into the middle of 3 trygons and run the whole lot forward... its funny watching peoples' facesthats what make venomthropes so awesome "Who gives a fuck about invul EVERYTHING IN MY ARMY GETS A COVER SAVE "