Gah, always turning up late to interesting discussions. Here are my thoughts, for what it's worth.
GW intend for you to play using their stuff for the most part, so it's arranged around that with a little thought given to other cases.
The buildings and ruins they sell and display in the rulebook don't have bases, or anything like area terrain incorporated into them.
To them, area terrain is generally something different, like areas of forest or rubble, or marshes. In any case it's a different piece of terrain.
Then going to the terrain set up rules, the way they suggest has the rule that terrain can't be within 3 inches of another piece. This is more than you'd think and means that rarely will you be in a situation where you'll be A) In area terrain, behind ruins AND B) In a good position to be offensive from this position.
Basically, ruins on bases aren't intended in the rules, which is odd.
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