1. I'm going to assume you didn't pull hate, everyone lived and life went on happily in the dungeon after you cast Bene.
2. Yes, technically Benediction pulls a lot of hate but, depending on when you cast it, it's really no biggie. Plus, you have hate mitigating moves like Fluid Aura/Repose/Shroud of Saints.
3. I DO typically save Bene's for the tank, but it is also our 'OH CRAP!' move...so use it when you think 'crap don't die on me!'
4. Going to assume the DD screwed up somehow for needing such a big heal in the first place...and then blame you, which is typical.
5. I don't think benediction can overheal...I think it can only 'heal to cap HP amount' but not sure,
6. On some speed runs/Holy Trash burns... I purposefully will use Benediction to heal the tank so I don't need to swap out of Cleric's Stance/stop DDing/waste MP. I can let the tanks HP's drop way low, pop bene and they are good to go.
7. Many DD's are completely ignorant to what the healers skillset is...or to the fact that we even have spells other than cure/aoe cure/sleep/shield type spells. They believe our sole job is to follow behind them like puppies and keep their HP bars full...which while based in truth, if it's all we did, gimps our role and party.

Final Thought:

How many times do DD's tell Healers how to play their role?
How many times have you heard a Healer tell a DD how to play theirs?