you only have to be within 12 inches from the engine to benefit from the wardsave. carnosaurs and CoC move 1 (2 if marching) inches faster than the engine, so it will take a while before they get out of the bubble, if you assume that they are reasonably close to start with. most scenarios have a battlefield with 24 inches between deployment zones, so by the time the CoC leaves the engine behind, it should usually be because they have charged in to combat. then they wont need the shield (as much) about the range of shooting. it protects from shooting fired more than 12 inches from the ENGINE, not from the TARGET. hence, if you keep the engine behind your front line (and that may be advisable in any case) it can easily protect your army from shots even if the target is closer than 12 inches from the archers firing at them. with 8th edition even allowing you to measure distances at any time, it should be possible with very precise placements of the engine to provide maximum protection for your units. as already stated, it is by no means a slann when it comes to magic, and i agree that heavens isnt the most spectacular school (remember 6th ed? lol!) but it can still work wonders with subtle augments/hexes and a comet never hurts either (well.... it DOES hurt... you know what i mean ). I think that an engine priest + a normal priest can provide adequate magic for an army. note that i said adequate, not great. if magic is your prime concern you should definitely use a slann, and then the extra skink priest you have to buy to put on the engine isnt that great. nothing says you cant go both slann and engine, but i think it loses a bit of its already doubtful value. But if you have no slann (and i know tournament players that goes that route, and still preforms) an engine can help boost your magic phase AND provide all the added utility of the engine itself.
I was thinking much more when you start getting into overruns, or when something tries to make a flank run against the enemy line. Let's face it, the speed of a Carnosaur combined with the sheer damage output is what makes it dangerous, and if its tied to a EotG that does slow it down some. There's also the risk of frenzy pulling the Carnosaur off outside of the twelve inch range, though that would be pretty unlikely since you'd get a turn to move back into the screen. You are correct that you could park the EotG back behind the lines, then you have a 265 pt model sitting there generating a 5+ ward versus shooting. I guess there are times that would be worth it but I just can't justify it in my head. Want the +1 to dispel? Pay 40 points for the staff and the other 225+ points on other things. Ultimately, I don't think the EotG is bad just not as perfectly matched with a Carnasaur list... though they do look awesome together. Pyre ...and on a total side note, this is perhaps the most rational forums I've been on in a long time. Thank you all for the discussion!