Free Step Dodge

Because it does no damage and it puts Kasumi in negative unless they're near a wall. Instead you can throw out any other throw and get way more damage that's actually guaranteed.

but what about grabing with her 236T after her 46T?

It would be more useful if Kasumi's oki weren't so good. But as it stands, Kasumi users aren't afraid to get the damage and knock you down, as they feel they can still apply pressure after doing so.

Why do 46T when you can just launch or juggle with 236T or 66T~T lol.

You can't 236T after 46T because the opponent gets pushed back and can now attack Kasumi.