Corvans can be super pesky in large games. They can always give you rerolls and as a patrol chioce you can field enough to be initiative sinks (though I appreciate some might consider this unsporting).
Even if your enemy does try to swat them they still have the stealth to deal with.
Its not a good idea to get them in range of your enemy's secondarys though. This makes their weaponary pretty useless (apart from as a poor mans AF).