Because it creates confusion. From what you say, one should maintain the "rollover account" and never mingle, until after retiring.
From Schwab's site:
------------------------ Can I combine my Rollover IRA assets with the assets in my other IRAs and still roll over my assets to an employer-sponsored retirement plan in the future? As of 2002, you can combine any assets held in a Traditional IRA, Rollover IRA or an employer-sponsored retirement plan like a 401(k),
403(b) or 457(b), including any after-tax employee contributions. But not every employer-sponsored plan will allow you to roll over combined assets. You should check with your new employer regarding their plan-specific rules. It is advised that you consult with a tax advisor for more details.-----------------------
This confirms your position. Disclosure - it's never about me being right, just seeking truth. I think I found it. The punchline is the new rule can confuse job changers into combining accounts, then finding the new employer will not accept the money. Seth - thanks for your patience in this dialog.
Joe