To fully understand why you should not combine fruits and vegetables while juicing, you may want to get a book from the library about food combining.
When you learn about food combining, you will realize that certain foods work well together and other foods do not.
Vegetables require different enzymes for digestion than fruits. When you combine the two, you create bad digestion. Since the whole purpose of juicing is to improve your nutritional intake, mixing fruits and vegetables together is not a good idea because you sabotage your digestion which in turn hampers your ability to absorb the nutrition.
The only exception is apples which have a neutral effect and can be blended with vegetables.
Saying that, when you juice, you want to focus more on juicing vegetables than fruits anyway. It's better to eat fruits since fruits are easily digested and most of the health benefits, such as fruits ability to cleanse the body, are only obtained when eaten whole. Drinking fruit juice is more a treat. Fruits are just too high in sugar to be juiced all the time.
One possible reason why others say that you can juice vegetables and fruits is because they are in the business of selling juice machines. As a result, they need to make juicing glamorous and delicious in order to sell their juicers. But if you are serious about juicing, you will quickly realize that the sugar content of fruit makes them unsuitable for frequent juicing. Why? Because a sugar is a sugar is a sugar, no matter its form or origin. All sugars, in excess, lead to sugar related health problems — hypoglycemia and diabetes are the two most common sugar related health problems.