I'll do what I can. In addition to the factors I mentioned initially, for someone in their 20's or 30's a 529 plan is a better place to redirect that extra $282 per $1K of tax deferred retirement savings. This assumes they have (or will have) children and they are in favor of a college education. The time horizon of when you will access those funds is more favorable.
But my real bias is based on the assumption that someone in the modest to moderate income range is often in a position of juggling cash flow. And it is likely they can't avoid debt to live their desired lifestyle. So in the case of someone who can manage of what remains after the ROTH (IRA or 401k) contribution, directing the cash into another bucket gives more breathing room. For someone earning $50K and choosing an 8% contribution, that $282/$1K is $1,128 per year. Earning maybe 6% compounded quarterly would grow to $6,522 in 5 years. That money would available to respond to emergencies, pay down debt, etc.
And in my system of mental accounting, I would have likely chosen a 10% traditional 401k contribution over an 8% ROTH 401k contribution because I'd see it as an instant 25% increase in my investment while still netting some additional available cash if I was in the 22% or higher marginal tax bracket. I would hope time and compounding would still leave me with sufficient retirement assets.
Best regards,
Chris