Massive extinctions

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Massive extinctions are big extinctions. There are five massive extinctions in earth history. All of them wiped out thousands and thousands of plants and animals. The average damage was around 65%, affecting different species. You could think this is too bad, but many scientists think sometimes it is necessary because the resulting empty areas are soon filled with new evolved life forms. Of course, filling empty spaces as well as evolution takes million years, but produces a great diversity of new life forms. The most destructive massive extinction occured 225 million years ago, and many plants and animals (around 90%) disappeared. But then, dinosaurs evolved filling all natural environments. Then they extinguished and mammals evolved to many forms, dominating all natural environments. What do you think? Are massive extinctions necessary? What would have happened if massive extinctions would not have occured?