Where did I say you don't need it at all? It should be in there. But the right sized o-ring works just as good as the OEM part. Maybe better, because modern buna rubber doesn't usually rot, or dry up like the OEM thing did.
In regards to longevity ? Come on man. The factory grommet has lasted since the 60's in thousands of cars. And still going strong. Lol
I just can't fathom why you would want to Half *** this ?
Its a .50 cent part. And No, an o ring does not work as well as the original part. Keep telling yourself that, but you are wrong.
Its a simple,safe,correct step, for your brakes.
Good luck.