I look at it this way.
When you first learned to read. Did you learn to speed read, or start with the basics?
Did you learn to sprint, or learn how to walk?
I also think it depends on how serious you are about the game. If youre serious about improvement, then i would lay off/limit the bullet. If youre in the game just for fun, then bullet yourself silly!
Now if youre an elite player like Nakamura, then it really doesnt matter.
Spot tactics quicker, learn to make rudimentary plans, see the board quickly, evaluate in a hurry. Spot weaknesses.
These are some of the things that can come from bullet but not from slow chess.
I wonder if this is just me or other players also feel that bullet makes them stronger