Is it worth Momentum Killing every boss?

[slydaa7]

So I just picked up the game and beat a few of the early bosses. What I noticed later on, is that there is an indicator for Momentum Kills in every bestiary entry. I didn't kill Morrin and Alpsy in Momentum Mode and I'm considering to start over because I'm a bit of a completionist.

Is there any reward for Momentum Killing every enemy once?
Can you refight bosses later on?
I'm assuming it's going to be VERY tedious to Momentum Kill every boss but is it ACTUALLY going to be VERY tedious?

[Doomerang]

Not really, boss exp is not that big of a deal. I don't ever notice getting more then regular fights with 4-5 enemies.