Depends on where you put slapping n the "pecking order" of essential.
Let`s use Will Lee as an example.He`s played on 1000`s of sessions,how many times was "slap" a pre-requisite?And even if it was,he has done the "homework"necesarry to make it a cakewalk.I.E. HE CAN READ LIKE A MUTHA,HE CAN TRANSPOSE ON THE SPOT LIKE A MUTHA,HE CAN SING WHAT HE PLAYS LIKE A MUTHA,HE CAN SING!!!!!!HE HAS A COMPLETE UNDERSTANDING OF HARMONY,HE KNOWS A ZILLION TUNES...ON AND ON AND ON....Point is,do your homework,learn your instrument in a musical fashion...be aware of the popular trends around you.
The mystery of styles and technique disappear,when you know the music behind it,either aurally or on the page.
Be a musician who happens to play bass,not a bassist who can't happen to be musical.
Anybody who really knows this instrument,as a muscal instrument,does'nt ask this question.
"Is Slap Essential?"