The MB339's would have been extremely limited with no radar in what role they could have carried out from Stanley. Certainly the Vulcan raid was dramatic but of limited military value . It was however of a greater psychological value to the British as it forced an already tired and demoralised Argentine Army realise that they were against odds they couldn't counter.
It's interesting to speculate on what the Argentinians could have based there but obviously the Lear Jet flight into Stanley just prior to the war had produced a report which steared them away from fast jet operation.
Tactically it was the right decision for the Argentians as the Skyhawk and Super Etendard would have been distinctly a prime target for Harrier /Sea Harrier attack with BL755 sat at Stanley.