Eowyn,
Perhaps the view of non-genealogists is something you want to address - heaven knows, there is no shortage of them in the world! Until very recently, I was one, so how they think of a family tree is pretty fresh in my mind and also probably pretty representative.
It never occurred to me that there is such a thing anywhere as "my tree" or "your tree". I always assumed that the way the world is, is simply one big completely connected tree ... it's just that we don't know what all the connections are.
Bottom line is that having one tree is no more than acknowledgement of reality and is, therefore, an accurate model for how the world really is.
As a postscript, I might add that, having seen the "my tree - your tree" proliferation of replication of the same or divergent data for the same people, plus having seen several G2G posts from newbies who are posessive of "their" family, that only serves to underscore the rightness of the WikiTree way to me. I had a hard time understanding what all the emphasis on the "one tree" business was all about - it seems like a "given" ... that's about the only thing I was completely comfortable with from the start!!!