It is useful in situations where a PC would not normally be tactically advancing though. Perhaps they are exploring or investigating a warehouse building when they start taking fire from a catwalk or from the top of some crates. This is when the rule would be very useful. It gives the PCs a chance to quickly dive for cover when they previously had no reason to tactically advance or hide.
The characters can always take cover in their own turn and then proceed to use Tactical Advance. maybe they lose a half action this way, but they also lose a half action with Dive for Cover too (unless they have Leap Up). There is, like, one tiny fraction of combat where this special use may come in play: if the character tries to dodge the attack of an enemy at higher initiative than him, in the very first turn of combat. But even then, the character is hard pressed to use a normal Dodge that will negate the attack completely instead of relying on cover and wasting a half action for a "mere" +10 bonus.
I think you're missing another use for it. A character in cover only gains the cover bonus to the areas that are concealed, so a guy behind a wall wall gets legs, and maybe body, while his arms and head are exposed.
Dive for cover makes the cover bonus apply to any area, so a character who is already in cover but gets shot in the head can use it to drop completely behind the wall and gain additional armour against a called shot or lucky shot.
Yes, and makes called shots even more useless than they already are.