Taking this logic a step further, why don't you just start with a fly (or an ATM credit spread), get the maximum credit possible, and then adjust only when the market dictates?
Quote from Cache Landing:
..... I could tie up 50% of it on a 10-point FOTM bull put spread for a $0.60 credit. Or I could tie up 10% of it on a closer 5-point bull put for a $1.25 credit. I prefer the latter as the resulting gains are similar in the long run, but the latter protects me against the black swan.

The reason that I don't all the time has to do with SET as I mentioned before, followed by riskarbs suggestion of exiting the position with 15d left to expiry.