In my early years of accumulating information from previous family researchers, I found a large number of family group records with only the direct line ancestor listed as a child. So the pedigrees went back a number of generations showing only son to father and then to grandfather and so forth. As I gained more experience, I realized that this situation was quite common. People focused on and recorded only the "Direct Line" ancestor as if the other family members were unimportant or not worthy of being recorded.
Unfortunately, this bias is prevalent for royal lines and other quasi-historical pedigrees. Many times the wives are omitted altogether, as if they didn't matter. I fully realize that in some cases the wives were not identified in the existing records, but I found this tendency to list only direct line ancestors at times and in places where there were plenty of possible records to supply the entire family. I also found this genealogical narrow-mindedness when people were trying to "prove" a relationship to a famous person.
It is also altogether too common to open a file from a patron at the Mesa FamilySearch Library and find the same situation, generation upon generation where only one member of the family is identified. Sometimes the names are the same from generation to generation, i.e. William Smith with a father named William Smith and so forth. I submit that is virtually impossible to keep that kind of pedigree accurate. Without the confirmation of the names of other family members, keeping this same name situation correct presents almost insurmountable problems.
So the principle here is to search for everyone in every family. Don't play favorites. If you are going to do the job, do it right. In cases where the wife is unidentified there are huge resources for finding the wive's name. If you have your pedigree back to the point where you are copying a royal line or some other ready-made pedigree, then make sure you are dealing with real people and not someone's hoped for relationship to royalty. As a general rule, any pedigree going back before 1550 A.D. is suspect, unless there are substantial and specific sources accompanying the pedigree.
Listing just the male direct line is not the only way of playing favorites. One of my ancestors was locally very famous and influential in a large city. The trouble is that he had three wives at a time when that was legally suspect. Two of the wives were socially placed in the society but the third wife came from a small town and had no economic connections in the big city. So what happened? When the biographers wrote the book about my ancestors wonderful accomplishments, they conveniently left out almost every reference to his third wife. The authors did not even go so far as to acknowledge her three children by this famous person. Rewriting history to support our own personal world view is rampant in genealogical publications. Any irregularity in the relationships between the individuals can become an excuse to write them out of the history altogether.
But you say, "Oh, my ancestors wouldn't rewrite history." Well, the truth is that they probably did. One great-grandfather had the family tradition of being adopted. My research did not show any support for supposed adoption theory but it did indicate that the great-grandfather in question was the son of one of the daughters in the family out of wedlock. I haven't gotten any positive proof, but the age of the mother (grandmother) and her daughters seem to make that a distinct possibility. In this case, the true mother of the child has simply been written out of the historical record and any reference to the incident conveniently eliminated.
Many times researchers will choose one line to follow and ignore the remaining lines. I need to speak up in support of exhaustive research. Don't play favorites.