Here's an article that sums up my opinion on why it is okay to include unsourced family trees from ancestry.com in my research.
Basically: "Effective family historians...exclude no potentially useful source, and they trust no unverified source."
USUALLY people have a reason for linking people in their online family trees. Maybe the reason is illogical, ill-thought out, or otherwise erroneous.
Well, guess what? Maybe the original records themselves have errors. Humans make so many mistakes. How is it ever really possible to come to know the truth in genealogy?
Here's how: gather many, many records. Analyze the information. Consider the informant. Correlate the evidence. Realize that it is not the number of sources that state a fact, but the quality of the sources. Are they reliable? Are you reading a transcription? Genealogical proof is about gathering many records (including original records), correlating them, citing them, and coming to a written conclusion.
I do think it's generally better to start with original records when you can, rather than to look at others' online genealogies. You can unknowingly introduce a bias in your analysis when you see something presented as "truth."
There is a big family reunion on my husband's side of the family in June for the Challis and Wood families. One of the nights will be a family temple trip, and some of us are working on preparing some names from these families so that we can do temple work for them. I have been working on some Northamptonshire genealogy research, and a lot of it involves rechecking others' work.
I love all of my ancestors. What's nice about Czech in comparison to English genealogy research is that so many original records have recently been made available for free online. English records are not even close to as accessible online, but there are many, many people who have been researching in them for decades (centuries!). Basically, there has been a lot more time for people to make erroneous conclusions, and it's a lot more inconvenient for modern researchers to recheck what has been done.
Hip hip hooray for Czech genealogy!