Whether or not to include fiber, and to what extent, should be based on the ileostomates tolerance of foods. The intestine has a remarkable capacity to adapt. Digested food in the small intestine is quite watery, and after it moves into the large intestine, a good portion of the water is reabsorbed into the body. Most fiber is indigestible material (from plants) that acts like a sponge, soaking up water and increasing the bulk of the intestinal contents making matter move through the system more quickly.
In a person with a colon, fiber is essential in preventing constipation and keeps a person "regular". This is the main function of fiber. Another theory about fiber is that it promotes mucosal growth, thus keeping the intestines healthier, promoting gut function.
Usually a person without a colon (ileostomate) doesn't have a problem with constipation and may have mostly watery stools or diarrhea. Again, over time, a person may adapt, especially if the last section of the small bowel (ileum) is still intact. So, consuming too much fiber, or too much "insoluble" fiber, may aggravate a person's diarrhea or watery stools. If this is the case, limiting insoluble fiber such as bran, popcorn hulls, seeds, nuts, skin/seeds, stringy parts of fruits and vegetables may be helpful.
However, another type of fiber (soluble) may be beneficial to the ileostomate. The function of soluble fiber is to make intestinal contents "thicker" and can actually prevent diarrhea. This fiber is found in oatmeal, barley, dried beans, peas, Metamucil, and the pulp of fruits and vegetables. Most foods have a combination of both types of fiber, but the above examples show the difference.