When dealing with actual gardens, a walled garden can be useful. I first read about the phrase as a metaphor for a counter productive practice on wikis (and, by extension, the web as a whole).
But nowhere can this counter-productive practice be seen more starkly than in genealogy.
Here’s an example: I discovered an (indirect) ancestor named Mattys Blanchan, in the course of looking for him on WeRelate, I discovered 3 pages for this person (and a bit more digging showed there had been a fourth one). Each one with slightly different names and different lists of children. Each one from a different person’s GEDCOM, each tracing down to different descendants. Each of these GEDCOM files represented a different walled garden, people labouring to put together a tree, not knowing that several other people were doing the exact same work in their own walled garden. Thankfully, by loading their data onto WeRelate, these walls could now be broken down and all these people could see that we are all cousins. But, sadly, that did not happen; in all four cases, the GEDCOM file was uploaded, and then they walked away (what’s known on WeRelate as a “drive-by GEDCOM”), leaving that work to be done by someone else (me, apparently).
That’s the good kind of walled garden: it ended up in a place where the walls could be broken down. But I have happened upon numerous web sites devoted to a particular family; many containing a wealth of information, but I have found few with sources or citations. Just bare, purported, “facts” with no substantiation. A garden full of things which could be healthy or deadly.
I should point out that I am relatively new to genealogy, and most of this is supposition based on what I have seen. But it seems that a tremendous amount of effort must be expended by people researching in their little “walled gardens” not knowing that many others are doing the exact same research, probably on the exact same ancestors. This is why I am doing all my work on WeRelate, it seems to be one of the few places dedicated to collaboration and quality research.