This prominence now seems to require a more specific explanation than that the author of the Fourth Gospel was previously JtB's disciple, and that the Baptist was a high status Holy Man and Prophet who could confirm Jesus's identity as the Messiah.
I now believe that John the Baptist was necessary to the ministry of Jesus; by which I mean that it was necessary that Jesus was baptised by John (and not somebody else) in order that Jesus could fully become the Messiah, could fully become both Man and God, could perform miracles (including raising the dead), and could have the self-knowledge to do all this in full awareness of its significance.
In other words, when John describes how he knew that Jesus was the Messiah because when he baptised Jesus the divine Spirit came down upon Jesus and stayed upon him - and John had previously been told directly by God that this would be how the Messiah was known - this represents a very significant and direct intervention in reality by God the Father specifically via John.
(Plus, John the Baptist's own miraculous conception and personal history, and its prior linkage to the lineage and life of Jesus, is described in the other Gospels.)
John 1: 29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
30 This is he of whom I said, After me cometh a man which is preferred before me: for he was before me.
31 And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
32 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.
33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost.
This carries the implication that in every other case when John baptised, the Spirit descended from heaven but did not abide - that the baptised person was 'touched' by the divine spirit - but not transformed by it into a qualitatively different being.
(This passage also confirms that the Holy Ghost came only with the ministry of Jesus, and that the many previous examples of divine Spirit intervening in the world - for example in the Old Testament, and before this point of the New, were distinct-from the Holy Ghost. I understand this to mean that the Holy Ghost was/is Jesus.)
This suggests to me that baptism by John was of miraculous nature for everybody - in being touched by the divine; but that this touching and abiding made a decisive transformation for Jesus. After which, Christ's miraculous ministry began.
In other words, John the Baptist's role in the incarnation of Jesus was not merely to help or assist; but was a necessary and decisive part of Jesus becoming what he became.
Now, perhaps if John had failed to do the baptism of Jesus (because John could not be compelled by God, he had to choose to do what he did, and for the right reasons), some other way would have been found - by God - by which the necessary and decisive transformation could be accomplished... This is quite possible, given God's power; just as perhaps God could have found another to bear Jesus had Mary declined.
But as it actually happened; I think we need to acknowledged that John the Baptist was as personally important as Mary - and indeed the analogy is a close one, since John's baptism was Jesus being 'born again' (as indirectly implied by the later discussion with Nicodemus).
Mary was responsible for Jesus being born as Man; John the Baptist for Jesus being born-again as Man and God - that's a measure of how important John was!
