Like for example, if you get a Luxknight, but you got him as an E, is it still good?
He is still good it just means he’ll get less bonus attack chance. :)
So is that a good thing or a bad thing
I would keep him cause he is a Luxknight, however E is the lowest grade you can get. With grades the higher you get the better.
k TY for the info 
Its not important though but there are ppl who would want the S grade only for their collection
Yes & No.
I’ve now collected all arks S grade, except A grade Fang & A grade Volvon (yes, for collection)
For pvp, 1 BA, might really determine the win/lose (So, no, not just for collection).
I say bonus actions are quite important. It can add floors for you in the infinite dungeon, and it’s certainly a game changed in PvP. Definitely go for S :).