According to Vanguard whitepapers, asset location(tax efficient investing) can add as much as 0.75% in terms of long term after-tax performance.
That is looking at a split portfolio invested perfectly tax efficiently VS the exact opposite.
What you are describing doesn't sound bad at all. Let me make sure I'm seeing it right though.
IRAs/401k: Target Retirement funds.
Taxable: Stock index funds.
Make it all match to accomplish desired AA.
Example: target AA 90/10.
401k: 500k in TR 2040
Taxable: 100k split 60/40 between Total Stock and Total International Stock.
AA = 89/11. Success.
That is tax efficient. The important thing is keeping tax inefficient investments(ex. bonds) in the IRA/401ks and keeping tax efficient investments(ex. stocks) in the taxable. You are doing that. You are also keeping all of the rebalancing in the IRA/401ks.
There are 2 disadvantages to this, and right now neither might matter.
1. Roth VS pre-tax: If you have both, and you have already put all of your bonds in the pre-tax then it makes sense to make the Roth extra aggressive. If you are using a pretty aggressive TR fund then this shouldn't make a big difference. If your pre-tax account and your Roth were both 60/40 that would be a bigger issue.
2. It works right now. One day it might not. As your AA changes and your accounts grow you might eventually get to the point where, in order to achieve AA, you can't use a TR fund just in the IRA/401ks.