I was able to find a lot of speculation that skipping crawling is bad, but very little actual evidence. Instead, I found studies that showed the opposite: there is no real difference between children who skip crawling and those who do not, at least in terms of other major developmental milestones.
I'm posting my answer from the related Skeptics.se question, which asked about specific claims that crawling allows children to develop past the symmetric tonic neck reflex, and that failure to do so could result in retention of this reflex, which could inhibit later development and motor coordination:
I was not able to find evidence supporting this theory. The closest I could find was a reference to a study in the same book cited in the article you linked:
The book "Stopping ADHD" cites a study by Dr. Miriam Bender that found that at least 75 percent of the learning-disabled people surveyed had an immature symmetric tonic neck reflex contributing to their disability.
It is impossible from this statement to identify any positive causality between lack of crawling and ADHD.
This paper suggests that it is lack of "tummy time", rather than crawling, that leads to retention of STNR, and that insufficient "tummy time" makes learning how to crawl more difficult and frustrating for the infant.
This paper suggests that the link between reflex retention and ADHD is not specific to the STNR, and that most of the typical ADHD symptoms are more likely to be associated with retention of earlier-stage reflexes (primarily the Moro reflex).
The number of children skipping crawling seems to be on the increase, and this is likely due to the movement away from allowing infants to sleep on their bellies in an effort to reduce SIDS (LINK). Note that a study referenced in that article found that there was no difference in other developmental milestones for the children who either learned to crawl later or skipped it altogether:
A long-term study of child
development, intended to follow nearly
15,000 infants from birth until
adulthood, began in 1990, just as
Britain began its Back to Sleep
Dr. Peter Fleming of the University of
Bristol, a director of the British
study, said that at first doctors and
parents were wary about the new
advice, and many doctors suggested
that the babies lie on their sides.
But gradually, as their fears were
allayed and data accumulated tying
sudden infant death syndrome to
sleeping on the stomach, virtually all
doctors began urging parents to keep
their babies on their backs. The
British study tracked this change. In
the early 1990's, when most babies
slept on their stomachs, they turned
over and crawled when the books said
they should. Within the last five
years, as parents uniformly began
putting babies on their backs, more
and more babies did not roll over or
crawl on schedule, and increasing
numbers never crawled.
But, Dr. Fleming said, the babies were
normal by every other measure. ''In
medicine, whenever you introduce
something new, you worry that it might
cause problems,'' he said. But, he
added, that did not happen. ''When the
cohort was 18 months old we looked
again at developmental milestones and
there was absolutely no difference in
these children's development,'' Dr.
Furthermore, this article suggests that crawling may have become a common developmental milestone only relatively recently, as leaving a child to crawl on the ground was frequently either unsafe, unsanitary, or both.
There seems to be little to no supporting evidence for the theory that crawling is a crucial process for moving past the STNR, and the problems cited in conjunction with late retention of the STNR seem to be correlation without causality. This is supported by the similar correlation with late retention of earlier reflexes, as well as the study showing changes in crawling milestone achievement did not impact other milestones.