It's an interesting concept, but if you can read through the Amerivest web site and understand enough to make an informed decision about Amerivest, then I think you can save yourself the 0.35% extra fee by doing-it-yourself.
Trading fees are cheap nowadays. For example, with $100K you can get $2.95 trades at WellsFargo. The $350 paid to Amerivest on a $100K account would be 10 times the necessary amount paid in commissions at WellsFargo. Think of the future as well. If you had $250K at WellsFargo, your trades would be free, but Amerivest would be charging you more.
And I disagree with your statement that ETFs are more tax efficient than Index funds. Where did that come from? I think they are about the same. Some ETFs have higher expense ratios than the corresponding index fund, some have lower; some may be more tax efficient, some may be worse.
Now if Amerivest charged a flat fee per year of about $50, then it might be worth it.