Almost total rubbish- this will work for destroyers since they are much higher then the scarabs but normal warrior sized infantry will give away cover when shooting through swarms- the rule stops MC/vehicles getting cover behind swarms (managing to hide 50% of either would be something to see) but nothing else... nids will still love you for giving their front rank a cover save (though any decent nid player should have 90% of his army in cover anyway).
in fact the Cover rule works in advance of the player placinf smaller units in front of the firing unit:
Originally Posted by "BRB p21 upper right
If a target is partially hidden from the firerís view by
other models, it receives a 4+ cover save in the same
way as if it was behind terrain.
so if anything of my NEcron warriors is blocked by a swarm I very well get 4+ cover
and as the cover is determined from the view of the firer it might very well happen that thw smaller unit that grants me cover does not grant your unit cover I am firing on
best example is the well known Gaunt shield or the Gretchin shield:
though the Tyra Zoa is "covered" by gauts its "viewing" point is so high any target it fires on will never get cover from the gaunts
same with Gretchin:
my Boyz and Nobz are big enough to see over the gretchin but will get cover as even if the slightes part of the model is covered by a gretchin blocking your LoS ... by RAW it gets cover.
long story short ...
Scarabswarms give cover esspecially if built as indended but do not block the LoS of the Warriors so the Warriors in most cases get cover.
Of course this is a very shacky problem and resolving the cover-situation is very hard as you have to count the models that are "covered" by swarms or Gretchinand then compare it with the Models further back in the target unit that are not covered ... but for ten Warriors I see no problem:
S = Scarab
W = Warrior
WWWWWWWWWW (unit 1)
WWWWWWWWWW (unit 2)
as you can see this 7 scarabs can provide cover very easily for the forst unit of warriors the following get theirs from the warriors in front of them ... now if you shift each warrior of unit 2 slightly left or right so that their LoS is not blocked by unit 1 and so again ... they might get very well cover but their LoS is not blocked so you don't get cover for them shooting through unit 1
it is cheesy somehow but very well legal