First, it may interest you to know that in Summa Theologica St. Thomas Aquinas "proves" that the dove the Holy Ghost appeared in was an actual dove and not merely Jehovah's sock puppet. See <http://www.newadvent.org/summa/403907.htm>.
As to why the HG is important, viewing religions through history it's apparently rare to find any that *don't* have some HG-like bird on the brain, viz.:
"The Christian’s Holy Ghost descended as a dove and alighted on Christ’s head at his baptism (Luke 3:22). The Holy Ghost in the shape of a bird – a dove or a pigeon – is a very ancient pagan tradition. In India, a dove was uniformly the emblem of the Holy Spirit or Spirit of God. A dove stood for a third member of the Trinity, and was the regenerator or regeneratory power. Compare this with Titus (3:5): regeneration and renewing of the Holy Ghost. A person being baptized under the Brahminical theocracy was said to be regenerated and born again, or, they were born into the spirit, or the spirit into them—the dove into or upon them.
"In Rome a dove or pigeon was a legendary spirit, the accompaniment of Venus, the emblem of female procreative energy. It is therefore appropriately shown as descending at baptism in the character of the third member of the Trinity. The dove also fills the Grecian oracles with their spirit and power. A dove was, in several ancient religions, the Spirit of God (Holy Ghost) moving on the face of the waters at creation (Gen. 1:2), though a pigeon was often substituted. The dove and the pigeon were used interchangeably.
"In the ancient Syrian temple of Hierapolis, Semiramis is shown with a dove on her head, the prototype of the dove on the head of the Christian messiah at baptism. At the feast of Whitsuntide, the descent of the Holy Ghost was symbolised in London by a pigeon being let fly out of a hole in the midst of the roof of the great aisle of St Paul’s Cathedral. It is more than likely that this continues an ancient tradition. On solemn occasions when the Holy Ghost was expected or invited to descend, it was more than likely that originally no one in the congregation noticed that it did. The custom therefore arose of liberating pigeons or doves at the appropriate moment.
"Naturally, these doves would have been actually ascending, having realised that they were no longer constrained, but that would not have bothered the faithful who eventually came to understand the symbolism. In any case, the doves would most likely have been tame ones bred for the purpose and possibly made no great effort to escape, like the pigeons in crowded city plazas. So, it is quite possible that sometimes one of the tame birds did alight on the priest – perhaps they were trained to do just that. The pictures of priests or gods with a dove on their head might be depictions of actual rituals.
"The Holy Ghost was the third member of the Trinity in several Eastern religions as well as the Gothic and Celtic nations. This notion of a third person in the the godhead was diffused among all the nations of the earth. Father, Son and Holy Ghost, or Father, Word and Holy Ghost (1 John 5:7) express the divine triad of which the Holy Ghost was the third member. The Holy Spirit and the Evil Spirit were, each in their turn, third member of the Trinity.
"In these triads the third member was not of equal rank with the other two. In the Theban Trinity, Khonso was inferior to Arion and Mant. In the Hindu triad, Siva was subordinate to Brahma and Vishnu. The Holy Ghost conception of the Christian world is an exact correspondence with these older ideas. It has always stood third in rank after the Father and the Son or the Word, a slave doing all the hard work and getting little worship for it. Today it is still seldom addressed in Christian devotion, but perhaps that is because it was so badly treated that it was not too diligent in its tasks. It was not too good, for example, at making the holy book of Christianity infallible."
<http://www.sol.com.au/kor/22_01.htm>
>alas, recently, on another list, it became clear that someone from a
>catholic background, and someone with theological (protestant) training,
>took this HG thing really seriously ...
That's the world of the faithful for you. One minute they're scrutinizing pigeons to see if God happens to be in the flock; the next minute they're launching an Inquisition to burn you alive. Funny folk!
Carl