Interestingly enough, TLOS can block a model or two from shooting without granting a cover save at all. Say I have 3 Tactical Marines firing at a single Dark Eldar Wych but there is a Terminator Chaplain(not joined to Tactical unit) between the two units. It is possible to have the models positioned in such a way that one of the Tactical Marines cannot see the target Wych. Therefore only 2 get to shoot but over 50% of the unit can see the target, with no hindrance, so there is no cover.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."