Well, a quality allrounder is a very useful addition to any side, but I wouldnt say that you really NEED one for a side to work successfully. If you have five quality performing batsmen in your side, and you are confident with them, then batting the keeper at #6 and playing five bowlers is perfectly acceptable. As is the reverse - if you have 4 quality performing bowlers in your side, and you are confident with them, then batting the keeper at #7 with a specialist batsman at #6 is also perfectly acceptable. Allrounders help this balance out somewhat, but they are not required. It all depends on what the strengths and weaknesses of a team are.
As for the specific Australian situation, Symonds simply isnt of test standard, and I have stated this for years gone by. Watson, IMO, will eventually be good enough to command a spot in the side, but hes not quite good enough yet. The selectors will pick him though, to get him used to test cricket and gradually give him more responsibility, just as they have done in ODIs.
Watson's the man - hes not quite ready yet, but he will be.