Is it really necessary to replace my timing belt at the recommended interval?

YES:

Timing belts helps your engine propel your vehicle down the road by determine when the valves will open as pistons move. It is part of the internal combustion engine. If you have an interference style engine and the timing belt breaks, the valves will hit the pistons and you will hear loud noise and the engine will cease to operate. The failure of a timing belt in many cars can result in major engine damage. The cost of repairing an engine with a broken timing belt is much greater than the cost of a timing belt replacement.