Usually it is said that a true all rounder in someone who will make it to his team either as a batsman or as a bowler. But I think a true all rounder is someone who may not be of any great value in either department individually on a regular basis, but would be irreplaceable as an overall package.
Players like Imran, Hadlee, Akram, Davidson, and Pollock, for example, could have averaged 0 with the bat but they would still have made it to their national teams purely as bowlers - like McGrath, Donald or Waqar - and be ranked among the greatest ever cricketers. (Who considers the 21 batting avg of Lindwall's and Marshall/Warne's 18 and 17 runs/innings while placing them among the very best cricketers of all time?)
Sobers, Kallis and Hammond - on the other hand - could have taken 0 wickets in tests but they would anyway have been bunched with Lara, Tendulkar, Border etc as the best middle orders batters of all time and still figure in the list of greatest cricketers.
But take Miller, Botham, Kapil, Greig, Flintoff, Mankad, Rhodes, Faulkner, Noble, Gregory, Cairns and Goddard. They all can point to some scintillating hundreds and fifties in their resumes that won games. Or five-fers and ten- fers that blew out the opposition. Even boast of out of the blue match winning catches. One of them scored 14 hundreds and claimed 27 five fers which is more than what 75% of specialist batsmen and 95% of specialist bowlers have achieved. Still, the USP of this men is so obviously their twin skills combined. They could have played as bowlers alone but very few of them would have made it to the top 30 list without difficulty. They would not have had long careers purely as batters (though it is always assumed that Miller was good enough to avg 45 in test cricket) And none would have made it to the top 100. But when you combine their skills, they all should be among the top 100 cricketers of all time, with some of them breaking into the top 20 easily.
In my opinion they are the correct examples for the term "all rounders". A great bowler like Imran batting so well or a champion bat like Sobers taking wickets should be considered nothing short of a boon and that is why they are among the top 5 cricketers ever. But if you want to define an all rounder, this group represents that term better.