Abs, like ANY other muscle group, adapt to the same stimulus over time. If you're OK with decent-looking, visible abs, then crunches, raises, twists and the like are probably sufficient. But if you want a well-rounded, functionally strong mid-section or "core" (stabilizing muscles all the way from your neck down to hips and everything in between) then you need to train all movements - flexion, extension, anti-extension, rotation, anti-rotation, isometrics and the Valsalva maneuver.
Another thing, if you always do high-volume ab training (15-30 repetitions), I'm very certain that you will notice a benefit and improvement in your abs' appearance and strength from cyclically incorporating heavier ab training (4-10 reps), which may be just what you need to take your abs to the next level.
And I'm just talking training, not eating habits.
Again, none of this is necessary to have a mediocre mid-section, but who the hell wants to be mediocre?