I don't get what the point of it is...it doesn't do anything, does it? Note that I'm talking about the one in Harkon's chest. So...am I missing something?

Where to is "Harkons Chest"?
I didn't know he even had one .

It's just a generic chest. You can find it in the same room as the final boss fight.

I didn't get anything in that chest .

I got it too...I dropped it and it just floats in the air and can't be used, can just be picked up again...the chalice was still in the castle though, I used it before I left. =P

I know, it's essentially an 11 lb Oblivion paintbrush! You'd think you could drink from it, or something...

Am I the only one that would use it to decorate one of my other houses? that would look soo sweet floating in my secret hideaway lab over in my Windhelm house.

Your not supposed to pick it up. Its given to you for one of the DG quests, you take it to redwater den and fill it with the blood of ancient vampires..Then you get multiple Radiant quests to purify it and you can use it every so often to boost your Vampiric Drain ability.

Hope this helps.