If they are in the 15% tax bracket now, they will be in the 0% tax bracket in retirement. 0% is much lower than 15%. If they are in the 15% tax bracket now, they are not saving gobs of money for retirement.
If they are set on a Roth, they can simply get the company match while saving on taxes by investing in the traditional 401(k), then contribute to a Roth IRA outside of the 401(k) plan.
I'm in the 25% tax bracket now. I will be in the 0% tax bracket in retirement.
A Roth 401(k) is great for folks who contribute the maximum to their 401(k)s and to their Roth IRAs and have a pension plan and who will work to age 70. That is, the wealthy folks. For who retire early and with no pension plan will usually if not always do better in the traditional 401(k). That's because they can still convert to a Roth after they stop working when they are in a lower tax bracket.