This is an excellent point, and it's really a key to Australia's monsterous success. McGrath and Warne are almost certainly the greatest pair of bowlers in test history. Whether or not individually they are the best ever at their craft is up for debate, but together they offer such incredible skill and diversity it is almost impossible for a team with both of them to lose on a consistent basis. As Mark Taylor once said, the most underrated aspect of Warne's game is his defensive abilities. He said that however good Warne was on a 5th day wicket with the opposition on the ropes, it was his ability to come on at 1/70 on a flat pitch on the first day and bowl 20 overs and concede 50 runs that set him apart. McGrath is a bowler who can bowl on any sort of pitch. He's absolutely lethal on a seamer, he's one of the best flat pitch bowlers ever seen and can extract life from any pitch, and he's really the complete fast bowler in every way except for pace. He can seam it, swing it, he's got a good bouncer, a yorker, a slower ball, an effort ball, he's about the most accurate bowler you could possibly hope for and he can even reverse it.
When you put those two bowlers together, and add their excellent longevity and match fitness and their competitve tempraments and the way they thrive when the pressure is on you simply can't have a bigger asset as a captain. Australia's team over the years has had some other elements, with some brilliant batsman, one of the best batting lineups ever seen in recent times, the once-in-a-lifetime brilliance of Adam Gilchrist and the underrated bowling support of Jason Gillespie, but really it's McGrath and Warne which took them from just being a good side and the best in the world to being an unbelievable side and one of the best of all time.