Fair point Elizabeth and in today's society, every transaction by our plastic cards, use of phone, every word on the computer, car we drive, address, National Insurance Number, bank details, insurance contents, type of accommodation, shopping choices and many others are already shared by certain people and I'm sure that major corporations have this information.
I could certainly think up a couple of different scenarios as to how family trees could be of use to others, but life and death goes on and people undertake genealogy for their own reasons and if I was a betting man, it wouldn't surprise me if some supercomputer could work out virtually everyone's trees anyway.
I can understand as to what you're doing and why you do it and although it's asking people to think twice about what they put down, I do appreciate your perspective and in my specific area of security technology, I am aware of covert computer systems (particularly in USA) that immediately analyse every person entering a shopping mall (for example) time on arrival/departure and able to immediately assess a persons weigh, height, colour, transaction, movement and even what they look at.
In conclusion, there's a multitude of different types of security software and systems about the place that is basically used to analyse people for different reasons, but everything that we do is recorded in some form or another as mentioned previously and the choice is down to the individual. You either accept it or not and if persons wish to pull away from such technology, then they should consider going the whole hog with some major life changing restrictions, or my personal alternative, is to just get on with what you want to do and don't worry about the what ifs or buts, as life's too short anyway.
Like I said, fair point and each to their own