re: Backdoor Roth question

1) If you are a high income earner you won't get a deduction for your traditional IRA contributions
2) Since you are already paying taxes on your earned income and you are not eligible for the deduction you might as well convert some of that into a tax advantaged retirement investment.

There is no downside unless you somehow don't have the cash to save. The money is protected against taxes at retirement anyway so worrying about tax rates at different life stages is a non-sequitor.