Interesting approach...I have actually traded diagonal spreads by initially purchasing the long leg and only selling the short leg after it goes against me to the point where I would have set an equivalent stop loss. In the cases where you get a runner, you're in a long option only, so you're good to go. Since my stoploss is usually fairly reasonable, the short leg doesn't lose too much value when it moves against me. In that case, I'm pretty much at square one...and in my experience, the benefit you get from playing those runners outweighs the losses you incur from letting the short leg premium drop a bit.

