If you just do all the big compound lifts the little muscles will take care of themselves, right?
Multi-joint movements like squats and pull-ups should make up the bulk of your training regimen. However, multi-joint exercises are suboptimal for training certain muscle groups.
Think of your hamstrings and biceps, for instance. Since they don’t change length remarkably during compound lifts, isolation is actually the best way to strengthen them. If these muscles are weak links for you, isolation is a must.
In conclusion, emphasize compound lifts, but also incorporate single-joint exercises to strengthen weak links and as “icing on the cake”.