For some genealogists, online family trees are anathema. I am guessing, but the number of genealogists who abhor online family trees is probably only an insignificant, tiny percentage of the "genealogists" who have their family tree online. With over 71.5 million members on MyHeritage.com alone, plus the millions more on Ancestry.com and other online family trees claiming huge numbers, such as Geni.com with over 77 million, it is more than obvious that online family trees are "genealogy" to most people interested in their ancestry.
So why the antipathy towards family trees? Errors and bad genealogy. Right now I am looking at my Geni.com family tree, for example. Here is a screenshot for illustrative purposes:
The arrow points to a suggested invitee to help "complete my family tree." The name, identified as a "great uncle" is Ralph Carum Tanner who is further identified as a "son of Henry Martin Tanner and Eliza Ellen Tanner." This claimed individual is well known to me because he does not exist. I have searched every possible record and no one by that name exists. I am also extremely well aware with elaborate documentation of each of the children of Henry and Eliza Tanner. He is not their child, if he exists. How do you get rid of this type of unsupported information? The answer is that this fictitious person now lives in hundreds of family trees online.
Apparently this core problem with family tree programs is no disincentive to the millions of online users. So, one of the most common reasons given for having your own independent desktop genealogy program is to provide a "clean" copy of your pedigree so that when the rogue online family tree users start messing up everything, you have someplace to go to get "good" information. This reasoning is based entirely on the idea that the genealogist making this point is always and completely right. What if each and everyone of these millions of online family tree users had all of their data in their own program. What difference would that make to the accuracy of the overall online family trees?
How would everyone having their own program on their own computer help with the accuracy of the existing online family trees? Wouldn't the desktop programs simply mimic the online family trees? I believe they would. The hard core genealogists take the position that they are right and all the online folks are wrong. This may or may not be true. Let's suppose I have my genealogy in three or four online programs. Isn't that back up enough? Why do I need another copy of the same information on my own computer? Especially if I don't know enough about genealogy to tell if there is a fictitious person in one of my families.
Technically, you could define genealogy to the point that your were the sole person on the earth who was a qualified genealogist. I think those who consider themselves to be genealogists are going to need a new way to "prove" that a person needs a local desktop genealogy program.