Some of the reasons outlined in this post are precisely why I have made my tree private.
There are people who start their family tree, who have absolutely no interest in 'truth'. Merely in going as far back as quickly as possible, often on the flimsiest of evidence or coincidences.
For instance, I've seen a member who's added family matching mine, who's only link seems to be that they possessed the same surname and were born in the same country. Never mind that for the link to be correct, a member of their family would had to have given birth at age 3!
I've spent quite a lot of time and money working on my tree, actually visiting records offices, combing graveyards the length and breadth of the island, spending a small fortune on certificates etc etc My tree does not currently go back very far compared to some, but every person in it is proven.
Send me a message, and I am perfectly willing to share the fruits of that labour where there is an actual connection. What I don't want, is people leapfrogging my research just to get further along their own tree when they're to lazy to do research themselves and aren't particularly fussed whether they've got the right people.