You are absolutely correct that an investment advisor is not at all necessary for those willing to do a little research and spend a little time tending to the portfolio each year. However, in my experience, most people are unlikely to do that. It seems the data bear this out as well. Despite having great tools at their disposal, most investors fail to use them or to use them appropriately (see the paper referenced in Nat's comment above).

We keep our clients disciplined by having discretion over their accounts. We make the trades, rather than recommending trades. That way, we know things get implemented. Clients do always have ultimate control, as they can fire us and do what they want with their money. However, for those who stick around, we do instill a significant amount of discipline to stay the course, rebalance, and make smart tax decisions.