My understanding was that it is good to introduce isolation exercises when you move from a beginner to intermediate. Beginner being someone who trains three times a week, with each day training all muscle groups (bench, squats, arms), to intermediate where you concentrate on one area per work out 3-4 times a week (arms one day, chest/back next day, legs/shoulders the next etc).
So when you go intermediate, you want to hit a muscle group 3 or 4 times a session. In which case you will need to add isolation exercises. Example: to hit the chest you would do a bench press, incline dumbbell press and then dumbbell flyes (the last exercise being an isolation).