While researching more things for my next book I came to the conclusion that Rod joining Maiden was the final piece Maiden needed to be successful because of his contacts. But I wouldn't say he's a HUGE reason for the success of the band. Just look at these facts.
By 1979 Maiden had been gigging for almost 3 years now and had already a pretty big following, they had the support of Neal Kay who not only had exposed the band at the Bandwagon (in a good night there could be 500 people there), he also exposed them to a wide audience (with his HM chart) and even had gone on to various record labels to have them signed. Also, because of Neal the band got exposed by the first review Geoff Barton did about the NWOBHM (which of course expanded by 100 the media exposure of Maiden). John Darnley from EMI had already seen Maiden even before Rod had come in contact with the band, so they were already aware of them and THEN is when Rod came into the picture...
What Rod brought to the band was his contacts. He was able to get them venues they were unable to play before him which, of course, exposed Maiden even more to other audiences and more record labels, however, as I said before EMI had already thoughts about contracting them (that why they had 2 tracks in the Metal For Muthas album) so that's what I see, at least at first, as Rod's contribution... Maiden did had everything they needed at the time and maybe it was just a matter of time before they were signed...