This is an excellent question and is one that I hear frequently from my clients and group fitness participants alike. To understand the answer, first you have to consider the main types of muscle fibers that comprise the abdominal muscle groups. Muscles of the trunk are primarily made up of slow-twitch muscle fibers that promote endurance. That is, these muscles are designed to support the spine and keep the trunk upright against gravity during waking hours. Thus, they need to be able to perform constant, low-force contractions continuously throughout the day without fatiguing in order to support posture. Because of this primary function, abdominal muscles can be worked effectively with low loads (e.g., bodyweight) and high repetition (e.g., equal to or greater than 15–20 reps).
There are times, however, when the trunk musculature is called upon to support or create explosive whole-body movements, such as in athletics or grabbing a small child before he runs out into the street. It is during these situations that the other types of muscle fibers (fast-twitch) are called into play. While not as proliferous as the slow-twitch fibers in the trunk, the fast-twitch fibers do exist and can be trained through athletic movements and strength exercises.
Because the primary function of the muscles of the abdomen is to support the spine both at rest and during all movements, these muscle groups are active and working during all types of exercises. In other words, the abs are working throughout each exercise session, regardless of whether or not the focus is on isolating them.
This brings us to your original question, which is how frequently should you train your abs? The answer is the abdominals need a day or two of recovery from focused effort, such as crunches and Pilates, just like all other muscle groups. However, if you exercise daily, you are still activating the abdominal muscles as they support the trunk during any type of movement. This is why if you notice your abdominals are sore from “traditional ab work” you might consider lightening up on other exercise loads or activities until the abs fully recover so that the spine can be supported without less risk for dysfunction and injury.