Toe-out causes instability, so there's no point in using it at the rear of your car, it will make it undrivable. But, in the front, there's the stabilizing effect of caster. That's why a little toe-out is sometimes used in front, as long as the car has sufficient caster, instability on the straights won't be a problem. The 'instable effect' will be noticeable though: while turning into corners. Turning in will feel more immediate and more aggressive.
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Toe-in at the front does basically the same thing: it stabilises the front. This can be a nice effect if you want to calm down the front of your car while accellerating. It will also make you lose a little steering; turn-in will be far less aggressive.