The future is not known.
To those that only use the Roth, there are a few issues to consider.
1) Taxes very well could go up from where they are today.
2) Just because a Roth is taxed now, and not taxed later on withdraw, does not mean that they will not tax you when you withdraw in the future. Yes, they probably wouldn't tax that money as income, but just may tax you on the capital gains. I understand that taxing the income now, and then taxing you an income tax on withdraw is double taxation, but it doesn't mean they won't charge you capital gains. Again, the future is not known.
By investing all your money into either or, you will either come out ahead, or come out behind. I find that splitting the two will not give me the best reward, but I also won't come out behind. Diversified is rule number 1 in investing.