For the average person in the 25% federal bracket their 401(k) contributions avoid this rate. When using it in retirement you need to go through whatever the prevailing marginal rates are at the time. As you do this you likely reap a benefit, I just don't see the lower levels of income being taxed at 25% or higher. The issue becomes what happens if the next bracket you're about to enter isn't 25% anymore. What if it is 30%, or 35%. Then you would like the flexibility to halt your 401(k) usage right at the marginal rate and start using the Roth instead.
It makes sense for a lot of people to have some Roth, retirement is likely to be a very long time and you could see many different tax regimes in place over the course.