Adding an upper body component to a lower body exercise will drastically limit the amount of weight that you will be able to lift with your lower body. For instance, combining a dumbbell squat with a biceps curl will limit the weight that are able to squat. This will significantly limit the amount of strength you will gain in your lower body AND core, will prevent you from increasing your metabolism and ability to lose body fat, and will hold you back from increasing your overall fitness. Furthermore, it will make it far more challenging for you to focus on using proper form, which is vital if you are serious about achieving the best results possible AND staying healthy.
This is just one of many examples that I often see, and the offenders are frequently females (or their trainers are making them perform these exercises) as they falsely believe that this will help them ''tone'' their muscles and lose body fat. Wrong!! Focusing on a single exercise and with a heavier weight is far more effective.
Many people also perform these combination exercises as they believe this style of training will cause the heart rate to be more elevated, therefore promoting greater cardiovascular benefits and fat loss. From a cardio and fat loss standpoint, it is actually more effective to perform a superset or tri-set of compound exercises, where you are focusing on using heavier weights and are going from one exercise to the next with little to no rest. Try performing a set of heavy squats immediately followed by pull-ups, and then push-ups, and tell me that that your heart rate isn't through the roof!
To sum it up, in order to get stronger, more muscular (or ''toned''), leaner, and fitter, avoid performing the combination exercises where one part of the exercise is impairing your ability to lift heavier weight on the other component of the movement, and I won't even go there when it comes to performing this style of exercise AND on an unstable surface... There are obviously exceptions to this rule, and I am not talking about Crossfit-like exercises. ''Meat and potatoes'' exercises might not be very showy, but they are WAY more effective than the other trendy exercises. As Leonardo Da Vinci once said, ''simplicity is the ultimate sophistication.'' He is absolutely right!