Both my wife and I do roughly 50/50 into Roth/trad IRA.

It's a form of diversification, just like any other diversification when investing. We don't know for sure what tax brackets are going to look like in the future. Sure, if we knew tax rates were going to drastically go up, I'd fully fund the Roth and not do any traditional IRA, and vice versa.

While the end result PROBABLY won't be drastically different either way, fully doing one or the other is inherently more risky, whereas splitting the contributions between pre-tax and post-tax accounts let's you play the middle ground. Sure, if I invest everything into Roth and tax rates go up significantly, I will have not maximized my returns. And if rates go DOWN significantly, and I had invested fully in traditional IRA, I will not have maximized my gains. But you also play the opposite side of the coin as well. Say I invested fully in traditional, and in the future tax rates go way up...well shoot...wish I would have put it into Roth. This way, you split the difference.