Put another way, you want to think about asset allocation for _all_ your assets, your entire portfolio. Then think in terms of taxes advantages. So if you want to buy bonds, think of what percentage of all your assets (including your spouse's, including retirement accounts, etc.) you want to be bonds. Then try to have all the bonds in retirement accounts since they get taxed at a higher rate than dividends.
Sorry if this is explaining something you already know.
So, for example, if you wanted the Wellington, you might instead buy all equities in your taxable account while adjusting the proportion of bond purchases higher in your retirement accounts.
Of course, psychology matters a lot. If you're planning on putting extra cash on hand into the taxable account rather than automatic investment, you'd have to recalculate the proportions each time which would require more effort. And you want the process to have as few barriers and require as little thinking as possible. Or if you just know you're going to think of the taxable account as a separate thing, you might want it to be less volatile so you aren't tempted to mess with the investments when the market swings. A 'set it and forget it' approach is key to success for most people.
