I share the frustrations expressed but here is my perspective. My tree at GENI allows me to get notices about new possible connections and comments upon existing ones...I don't "use" it as my main reference tree and thus I am not horrified if someone alters it. (In a few instances, I just altered it back!) The tree I rely on personally for myself is Ancestry (where I periodically download it to my hard-drive as a back-up) and that one can't be messed with by anyone else. As for folks who erroneously copy from it and attach sections to their trees, I write them a short note at ancestry (click on their name when you come across their tree) explaining why the connection is impossible. If they ignore me...it's really their loss---I don't worry about serious genealogists (people with significant trees) glossing over my tree because it differs with another. In the end, I get more benefit from trees on Geni and Ancestry etc. than the "privacy/security" of not having them is worth.
Finally, when viewing the trees of others and deciding about "absorbing" later generations they have that I don't, in addition to records etc.---including looking people up in the Jewish Encyclopedia or in Jewishgen databases etc.--- I use common sense. If the names and flow seem logical but dates are off, I will adjust dates (ca)...if dates are ok but names/flow seem off, I just don't add them to my tree or I include (SPECULATIVE) as part of the surname....this is particularly true when a somewhat famous person (rabbi for instance) has a zillion children...obviously folks are just adding an ancestor as a child so they can be descended from that rabbi! Oy veh. Usually an internet search on the rabbi's name and dates gives enough clues as to which children and locations are believable and which are not. And what I'm usually interested in just the one or two children that clearly connect to my lines so there's no need to add the siblings until and unless they become relevant.
aka Debra Katz
Pacific Beach CA USA