A fifth front line bowler is an extremely valuable asset for any captain even if you assume an outstanding bowling attack such as an all time WI or SA team. To take Cricinfo's selections, for example, the WI attack would consist of Marshall, Ambrose, Holding, Sobers and Gibbs. One of the main objectives of any captain lucky enough to lead such a team would want to keep his fast bowlers fresh, using them in short spells so that at least one of them is always available. Gibbs, the spinner, would be the stock bowler, bowling very accurately on good wickets and becoming a potential match winner if the pitch takes spin. This approach can work only if you have a fifth front line bowler to do the unglamorous but necessary tasks of bowling into the wind and with the old ball. In this attack Sobers would probably get through as many overs as the fast men. Without him a captain would be forced to use at least one of the fast bowlers as a stock bowler rather than a strike bowler, and that would reduce the attack's effectiveness significantly.
The same considerations apply to an an all time SA team, the only difference being that South Africa, due to their wealth of all rounders, would have a sixth front line bowler, the leg spinner Aubrey Faulkner. A captain would want to keep Donald, Shaun Pollock and Procter fresh by bowling them in short spells, with Tayfield as the main spinner. Kallis as the fourth seamer would come on as the second change bowler, bowling into the wind. Due to Faulkner's presence he probably would not bowl quite as many overs for SA as Sobers would for WI, but - especially on pitches favoring seam bowling - he would be an important member of the bowling attack. It's much, much better to be able to call on someone like Kallis as a fifth bowler than to have to rely on batsmen who are at best occasional bowlers.
Even though it was against India
