Tax deferred until when? Something like this can be very valuable if it allows you to defer income from a high-earning year into a year when you're in a lower tax bracket (like the year you retire, for example). Otherwise it just doesn't seem all that compelling, especially if the investment options aren't very good.
The compelling things about deferred taxation are:
1) arbitrage - timing withdraws to occur so that they are taxed in a lower bracket in the future than they would be now,
2) growth of deferred taxes - i.e. if I don't pay 20% in taxes, my account has 20% more appreciating assets in it. This is like taking a free loan from the government for years and you get to keep the appreciation.
Of these, #2 seems worthwhile on its own. If the OP defers taxes (let's say 20% bracket) on $100,000 of earnings, for a nice round example, s/he is putting $100k into this non-qualified account instead of $80k into a after-tax account. If that extra 20k earns 5% in each of two years, that's $2,000 in earnings. That may not seem like much, but look at the risks people are taking to get an extra 1% these days!
Regarding tax bracket arbitrage, maybe one could time their FIRE date so they have at least $23.5k earned income in the calendar year they go FIRE. Then, make a 18k contribution to your 401k plus another $5,500 for your traditional IRA, maybe right before quitting. This strategy might further defer part of the tax impact of taking that lump distribution, AND keep the rest of it in a lower bracket. It beats quitting in December and taking your taxable lump sum distribution before end-of-year for sure!