Dieting and Food Allergies
While these "blessings in disguise" for the dieter do exist (and there are many more), the downfalls to dieting with food allergies could most likely be found in two things: difficulty with adhering to a specific diet (i.e. South Beach, Nutrisystem, Jenny Craig, Weight Watchers, etc.) and the consequent necessity for more time-consuming planning and strategy.
However, as food allergies become diagnosed at a greater frequency, more and more diets or dieting "agencies" are making room for individuals with food allergies, and more and more companies are creating alternatives for those individuals. Many local chain grocery stores and restaurants now advertise products that could only be found at organic food stores just a few months ago.
All in all, any individual with food allergies looking to lose weight should be consoled that they have the lifestyle, mindset, skills, and self-control from necessity that are demanded for a non-allergic dieter out of conformity. Their road to health and fitness will be much less difficult. It simply requires a greater refinement of the strengths they have already set in place.