"most of the people who are willing to pay to put their tree on Ancestry.com are serious about genealogy"??!!
Most of the people who put their tree on Ancestry (I am talking about the more recent members) think that is the way that you do genealogy. You put in your parents names, click on a few floating leaves, and there you have it - your genealogy is done! And they also think that Ancestry is the only place to find information, because no one has ever told them differently.
I personally have not put my tree online and won't. I am happy to share information with other family members, but most of what I find online, is either from a person who is the husband of a fourth cousin and only related by a thin string. I have found very little useful information from other people's trees. My most rewarding contact was from a cousin I came in contact with through a message board. We corresponded for two years before finding the documents we needed to prove we were related. Even if I found information from a tree online that was my family, I would never download a whole GEDCOM, but pick and choose what information was useful and verified. I found one family I thought might be related, but they insisted we weren't because they had never heard of my gg grandfather. Turns out not only are we related, and I found a relationship that they were looking for, but they have not ever responded to my attempts at contact, nor have they corrected their tree, even when I gave them sourced information. So, I have let that one go. Anyway, that is my longwinded answer to why I don't put a tree online at all, and I don't feel I am missing anything by not doing so.