Simple: let us say, hypothetically, that you think the earth is about to be assimilated. You are the captain of a starship in the midst of the battle, and you realize the billions of lives on earth are more important than the hundreds on board your vessel. You have two logical reasons not to make a suicide run:
1.) You still think you have a chane to win without destroying your own ship.
2.) Your ship is damaged, and you cannot make a suicide run.
(3. A suicide run is so against your idealogy you'll doom billions so your crew can have somebody else kill them slightly later - not a logical reason.)
We know that the Borg, if you've ever seen First Contact, do not concentrate their fire on just one enemy ship to destroy, so by the time a kamikaze attack seemed like a good idea, it was probably too late for that judgement call to be made - none of the battered vessels left intact were capable of warp.
You'll notice that as Acting Captain, Commander Riker did order a collision at warp two that was belayed at the last possible moment.