Is it necessary to train abs if I do many compand exercises that work the core too such as deadlifts for example?

Yes, I think so. Novices don't need it but I believe lifters who are no longer novices should include some direct ab work.

Here's why:

When doing the big lifts, your abs are primarily involved in stabilizing your trunk. But what about developing them to resist motion from other planes, or to produce motion?

Powerlifters may not need to develop the abs for producing directional force, but throwers will. And I believe lifters will benefit from training stabilization in other planes besides the sagittal one.

From my own experience, I can tell you that strong abs are important to anyone with a back injury history.

Also, have you ever noticed that many good squatters are, well, squat of build? They are thick and wide. That bulk gives them great trunk support. But slender guys will have a harder time supporting the same weight. They will have to do more direct ab work to make up for it, along with developing their back strength.

The real trick to squatting and deadlifting big weights safely is to have a strong "core". Your torso is primarily the transmission in these lifts while your legs are the engine. If your transmission is loose, you will lose force that is supposed to go into the barbell, and you will also be more susceptible to injury. But if you have strong abs and back muscles, it is much easier to support the weight, so it feels lighter and it's easier to focus on executing a heavy lift correctly rather than merely trying not to get crushed.

Personally, I like to do one light ab workout and one heavy one. The light workout consists of chops and the like for high reps while the heavy one is usually a standing cable crunch. You can do this exercise to train stabilization rather than force production by pushing your hips back into the directional force, but throwers will. And I believe lifters will benefit from training stabilization in other planes besides the sagittal one.

From my own experience, I can tell you that strong abs are important to anyone with a back injury history.

Also, have you ever noticed that many good squatters are, well, squat of build? They are thick and wide. That bulk gives them great trunk support. But slender guys will have a harder time supporting the same weight. They will have to do more direct ab work to make up for it, along with developing their back strength.

The real trick to squatting and deadlifting big weights safely is to have a strong "core". Your torso is primarily the transmission in these lifts while your legs are the engine. If your transmission is loose, you will lose force that is supposed to go into the barbell, and you will also be more susceptible to injury. But if you have strong abs and back muscles, it is much easier to support the weight, so it feels lighter and it's easier to focus on executing a heavy lift correctly rather than merely trying not to get crushed.

Personally, I like to do one light ab workout and one heavy one. The light workout consists of chops and the like for high reps while the heavy one is usually a standing cable crunch. You can do this exercise to train stabilization rather than force production by pushing your hips back into the cable column, which will cause your torso to lean forward without actually doing any spinal flexion.

The bottom line is stronger abs will always help.