I think some people are not getting the point that when you are in an 11 plus area, not putting a DC in for the 11 plus is not opting out of the selective system.
So to illustrate very crudely, let us take 100 children across the ability range from 1 to 100. In a comp area, all 100 go to the same school. The very bright children (say 1-20) are streamed into one class and benefit from this in terms of pace, peer group etc,. The children in the lower streams are taught at a pace that suits them, but they still have the same teachers, resources etc. If child number 40 is a late developer he/she can move to the top set.
In a grammar school area, however, children 1 to 20 are streamed into a separate school rather than just s separate class. (In reality it is probably not 1 to 20 but maybe 20 out of the top 25, given some will under-perform in the test like Seeker's DS, some choose not to sit it, move into the area, different type of intelligence etc).
Therefore opting out of 11 plus is not opting out of the selective system because child A who ends up in the high school is with children 21-100 (I appreciate too crude a way to describe children), and although there may be a few other children who would be in the top stream in the comp, the top set in that school is on average well below the top stream in the comp. which inevitably affects the pace at which that set can be taught, and there will be different teachers, facilities etc. Unlike the comp, child 40 does not have the ability to move to the top stream as it is at a different school
SO what Seeker is saying is that she would opt out of 11 plus if DC could go to a comp with children 1 to 100, but opting out of grammar to go to high school with children 21 to 100 is not the same thing. It has the downsides of the selective system and so is not electing out of the consequences of the selective system.
Sounds perfectly logical and consistent to me.