They're not necessary at all.
After a hard day's worth of compound lifts and stretching EVERY muscle after a workout, with the optional jog or skipping of the rope, my abs hurt the next day. And what's that? NO DIRECT AB WORK?
AWWWW
I win.
You can risk your back's health, but you will get little to nothing from direct ab work.
It's the one part of the body that's used constantly to stabilize your core, it doesn't need any other pressure added onto it than just that - STABILIZING YOUR CORE, which is done in every main lift.
I know many people who've had back problems from ab work and people who haven't.
But I say don't test your spine, especially when you get nothing out of doing so.
It's like cutting yourself just because you can, you don't even get pleasure out of it. So, why?