We exceed all income limits on either Roth or Traditional. So then maybe a better question would be:
Is funding a backdoor Roth a better use of funds than contributing to Index Funds now? With a Roth, we'd still be subject to age limits for withdrawing without a penalty, for one thing. It seems the tax-free dividend growth is really the only benefit...
Also, please be aware that a Roth IRA is just a bucket you put money into, not an investment as such. You don't have to choose between an IRA and index funds. You can put money into an IRA and then invest that money in an index fund.
As to the taxation piece, tax-free growth is a great thing, especially if you plan to be working for a while and/or you don't expect the 0% dividends/capital gains rate for lower tax brackets to last forever. Backdoor Roth contributions can generally be withdrawn at any time with no tax or penalty due, it's only the earnings that are more constrained until "normal" retirement age. I tend to think that constraint on withdrawing the earnings is well worth the benefit of tax-free growth. Everyone's situation is different though.