"Why would anyone want to add their family to the wiki tree on FamilySearch.org?" I asked in my original post that started this thread.
Well, it didn't take me long to realise it doesn't matter whether I want to put my family tree on the FamilySearch wiki tree -- it's already there! On mum's side I had to go back several generations before I found anyone. They were there because FamilySearch has added all the records it holds to its tree. My paternal side was a different matter - my late dad, his deceased sister and my grandparents were already in the tree, courtesy of a second cousin sharing.
According to James Tanner, of Brigham Young University, in a webinar about the Family Tree (because there is only one), "every person who has ever lived on the planet will have one identification number". It is up to genealogists / family historians, to find the duplicates and merge them, (preferably) giving sources to justify their actions.
I have not yet been able to find a person added by Family Search that has two records, therefore duplicates must (and do) exist. For example, I have found a marriage record for what I'm assuming are my 4G grandparents - "assuming" because I have not yet proved it. At the same church, within a few years, are what I assume are the baptism records of my 3G grandfather and his siblings. The parents are, of course, listed on the baptism records, yet none of these baptism records are attached to the parents, instead duplicate parents are created! Do I want to go through and fix all these duplicates? Maybe, one day, when I'm more sure of my facts.
Living people do not show on the tree except to the people who put them there. So, just because I can't find myself (my mother, my siblings, my husband, my children and grandchildren) on the tree, doesn't mean some cousin hasn't put me/us there - it only means he/she can see me/us but I can't!
Moral of the story, if there is one (because somebody may be as surprised as I was): some of your family data is probably already in the tree in some form - either because FamilySearch holds records relating to your ancestors or because some cousin has added your mutual relations.
I think the concept of a "one world tree" is a fascinating idea but I'd want more control over it than the wiki format allows and, more importantly, I wonder what the hidden agenda really is. James Tanner seeks to comfort the concerned by stating that the LDS church will never sell your information -- but why are they collecting it in the first place? Interestingly, genealogy is part of the "mission training" of LDS members. Perhaps I'm too sceptical - perhaps, because genealogy is important to them, they are genuinely trying to connect family members?
Something to think about perhaps, but I'm not going to let it concern me.