well, 99.9% of the time all those wee gribblies will be within synapse range so pinning isn't an option!
and even then, with the instinctive behavior rules coupled with a paltry Ld5, you're a bit foolish to try and pin the gaunts as they'll likely either lurk, (and thus not count for scoring unit purposes), or else they'll run off!
instead, target things like genestealers or gargoyles who might end up outside of the synapse web - they can be pinned! (albeit they both have Ld10, but they still need that IB test to move so eventually, they'll fail something!)
cheers!