The threat to a buy-and-hold program is the investor himself. Following his stocks and listening to stories and advice about them can lead to trading actively, producing on average the inferior results about which I’ve warned. Buying an index avoids this trap.
The #1 benefit of a good advisor is behavior modification. If you have somebody who is more disciplined than you, sitting at a level of detach meant from your finances, you think twice about discretionary purchases and trading.