Squash, AIM is essentially a hedging technique. It doesn't fare well with buy and hold in strong bull markets. How it does depends on the volatility of the stock selected, and how that stock performs in the future. Since only a few of us have crystal balls, AIM helps us to buy from the scared, and sell to the greedy, whatever the market cycle.
Further, when we say AIM underperforms B&H, we are usually talking about total portfolio growth. If you just look at the dollars actually invested in the stock, I think you will find AIM did very well. In my long term studies of the Dow, I find that AIM underperforms because it just builds up too much cash. You might consider Tom Veale's 'Vealie' as a way to help in this area. I've chosen to keep my cash reserves low, and invest in less volatile income producing stocks (though in recent weeks, they've proven to be volatile). I use these assets as a backup cash reserve. Another idea would be to invest in several asset classes with low correlation (or even better, inverse correlation) and share a common cash reserve. You will find that almost everyone here uses a variant of AIM, I've lost track of how many different permutations there are.
Also, for many, AIM isn't really automatic. Most still use their own judgment to override AIM at times.