Well, that's the big question, isn't it? The F-14 isn't an obvious fit for South African strategic requirements. It was designed as an interceptor, able to shoot down Regiments of incoming Tu-22s from Soviet Naval Aviation before they could launch their salvoes of shipkilling missiles. That's not a situation South Africa ever had to worry about, so they won't be getting it for its utility as an interceptor.
The F-14 also turned out to be a pretty good air-superiority fighter in general. South Africa's enemies and neighbours did have air forces, but I don't recall them ever taking much part in the various conflicts, and if they did the existing SAAF inventory could handle them quite competently.
That leaves the F-14A's strike capability (bad) or use as an AWACS platform with it's powerful radar (arguable). The South Africans did have a use for those capabilities, but they also had easier/cheaper/better options for them.
Against all that, you have an incredibly expensive, maintenance intensive aircraft, which requires two highly-trained crew, and is operated by precisely one other nation (who is entirely likely to cut you off from spares and stores if you upset them). To me, it looks like a bad deal. South Africa would get much more benefit from another squadron of Buccaneers, or airborne tankers, or multirole fighters like the Mirage F.1. If you want to make their ground forces happy, you could also look at increasing the numbers of helicopters they had, and maybe get a squadron or two of AH-1s. The navy would benefit from more patrol aircraft - the Shackletons were a bit long in the tooth, but even a F.27 Friendship with a radar would be a useful asset.