I think they do keep ppl in mind but there are three factors that make it hard to make rules that are totally abuse proof.
One it is very very hard to improver your own writing cause you know what you mean. And every time you read your own rules back your are going to see that meaning
A other thing is that there are literally millions of players so when a book comes out a group properly 100.000 time larger then the test team go in and try to break the game (cause that is what gamers do). Then we scream and rage about it on the inter webs and in no time everyone knows about it.
OF course GW could try to hire more ppl but that would be insanely expensive and the game is also a bit to big to do a open test whit the comunity that would just generate a completely clusterF of data that nobody in a right state of mind would want to take on.
Third think is Hindsight is 20/20
Also to every one who thinks they can do better. I dare you to come up whit a rule for a unit that needs to cover a nice fluffy interaction
Then post a topic about it and see how many ppl find a way to break it.