Mark A is right. First, you don't know what will be the most highly returning asset class. Second, given identical long-term rates of return, you can have a greater total or cumulative return by owning several asset classes AND you will have reduced volatility.
Investment 1 returns 40% in Year 1 and -20% in Year 2, for an average annual return of 10%.
Investment 2 returns -10% in Year 1 and 30% in Year 2, for an average annual return of 10%.
Having a 50/50 split of both (let's say growth and value) will provide for an average annual return of 10%, but you would have had a smoother ride (15% in Year 1 and 5% in Year 2). Plus, your total return in that portfolio would have been greater.
Note: Investment 1 had total return of 112%. Investment 2 had total return of 117%. 50/50 split had total return of 121%.