Again, i am not fussed if its Ivanovic or not, but i can see the logic to signing a proven quality defender for reasons stated previously, most defenders in their prime wont be joining Albion, thats why we have the likes of Ajayi, Kipre and O'Shea who have done well and now have the chance to step up.
For what its worth, a few of my closest mates are Villa fans (i know, i know!) and they all said the moment John Terry went to Villa, he just settled the defence down, the panicking stopped, it was when Johnstone was on loan there and they said he looked a quality assured keeper with Terry infront of him compared to the nervous one he had been before.
Rightly or wrongly it looks like Johnstone is number 1 so maybe Slaven has seen with the current crop he is nervous and wants a proven leader and organiser in the defence, even if that player will have a few miles on the clock.