Err, why wouldn't he make the side as a pure batsman? Watson's first class record: 2657 runs @ 45.03 with 8 centuries, as well as 72 wickets @ 28.73. He's probably not right in line for a batting spot just yet, but with a FC average of 45 he's certainly not that far off, and his bowling adds to his appeal as well, which is why he's going on the Ashes tour.
Keep in mind that he is also only 23, and therefore still well short of his peak as a player. And sorry, but if you're going to suggest to me in any serious fashion that Afridi is a) a good all-rounder or b) a better player than Shane Watson, you are nuts. Afridi is neither a good batsman or a good bowler, he is passable at both and has moments of sheer brilliance with the bat followed by long periods of mediocrity, while with the ball he is a handy part-timer at best. He's a very good ODI player, but a poor choice in tests, and would NEVER make a test side as a batsman alone.