Actual all fours crawling is important for other development, learning etc later in life.

While it may not be unusual for a baby to skip this and go straight from commando crawling to walking, it is still important that they do the all fours crawling at some stage as well.
My GP said that this is actually not true at all and that some babies skip crawling altogether and it has no affect on them whatsoever. I asked her because I had also heard that it was important.