Hell all it takes is the salvo fire form a ravana to kiss your corvette bye bye. Rememeber that annoing fast recharge rate that shivan beams have? by the time your frigates or corvettes get around to do any significant damage...you lose almost half of your forces.
Ah, but remember, while the Ravana is busy frying Corvette 1's ass off, Corvette 2, Corvette 3, Corvette 4 and Corvette 5 are buzzing around Mr. Ravana's back yard pummeling him with TerSlash beams. What we're trying to say is that a superdestroyer can only be in one place at any given time (unless its in pieces), but a corvette swarm/group/wing can be in more than one place at once. They're much more venerable, and are harder to disarm. The corvettes will cover each other's back, while a single superdestroyer can have blind spots.
A SD gooing into a sistem would have no problem in engaging a Ravana and taking out fast enough so that it would not pose much of a problem even if it has lauched 20 or more fighters/bommbers. It's own fighter/bommber and aaaf defences can take care of them. And even if you have half a dozen other warships gooing after the SD (corvettes and cruisers) they would be dispatched very fast by the superior firepower of the SD.
Ah, but could a superdestroyer alone take out a Ravana and its attending fleet who
are attacking different installations before said installations are destroyed? The answer is no. It would have to nuke Mr. Ravana, recharge warpdrives, attack Rakshasa 1, recharge warpdrives, attack Cain 1, recharge, Cain 2, recharge, etc. etc. A group/wing/swarm of corvettes would be able to destroy these threats simultaneously.
While a superdestroyer may have no problem destroying these threats individually, it would not be able to kill them if they were placed individually and not altogether (well, at least not as fast as a corvette group).
If you have a large fleet of ships all grouped together at a rallying point, then a superdestroyer would probably be the right choice. However, if you have five targets spread out over a system, a force able to divide and conquer would be the better idea.