There is no such thing as a "filler" node.
I keep bumping into lightweight wu's that people create to fill hardlinks in what they consider the "primary" node involved. I was guilty of this myself once, before wiser and better users here pointed out the error of my ways. At least I can savor the irony.
For example, let's say I was noding "car", and used the term "wheel", making it a hardlink. Following the link, I find that there isn't a wu for wheel, and decide to create one.
The problem occurs when the wu created relies on the node linked from to provide context. In the above example, if I write under "wheel" that it is something that holds up a car (hard linking the word "car"), it is a recursive loop that doesn't tell the reader anything. If I say that wheels are made of rubber, have metal rims, grooves for traction in water, etc, without saying that a wheel is a round thing that rolls and in conjunction with an axle can support weight, etc, I am only explaining features, not defining the thing under consideration.
This is related to the factual wu where the writer assumes that the reader knows the primary definition of the term in question, so talks all around the topic without actually saying what the hell the thing involved is.