To me there are obviously 3 types of all rounders, a authentic top orer batsman who bowls and supplements the bowling attack (Sobers, Kallis) . A bowling all rounder, a front line bowler who can also bat a bit and normally bats at 7 or 8 in a strong line up (Hadlee, Imran, Akram). Finally there is a true all rounder who bats in the lower top, and also a front line/opening bowler, but whom isn't truely GREAT at either, but very good at both (Botham, Miller, Dev)
With a batting allrounder you get good balance as you have a full batting order and a capable fifth bower who can give your bowlers a rest, once again a modern example being Kallis.
A bowling all rounder gives you good batting depth, but unless you still have a batting or true all rounder you still only have four bowers and less flexibility. Modern example being Stuart Broad (recent vintage)
A true all rounder gives that fifth bowling option, and a better bat than the bowling all rounder, but your still short a full out specialist batsman, but depending on your team make up this or the batting all rounder would be your best option.
Take an all time team for example, if you play Sobers as a batsman, and Imran or Miller at 6, you weaken your line up for a bowler, when you consider the other bowlers available might bowl at most 10 overs. If you play Sobers you have a full strength batting line up and for the role required behind your four front line bowlers of allowing for rest and rotation and keeping things tidy.
Most of the great teams had four front line bowlers and then some one who could turn their hand over for a few overs in a bind, five front line bowlers for most of history has been seen as overkill, and Australia of late 90's-mis 00's only briefly used Gilly at 6, but for the most part took their chances with four bowlers with The Waughs filling the void as a fifth bower. The West Indies of the 80-90's went with four with Richards, Hooper ect backing up and both teams did quite fine. The England team of today utilises Pieterson as their fifth bowler and they are currently #1.
It's a matter of preference and team balance.