You're helping me formulate my thoughts more clearly. Thanks. Before I answer your question, I should point out that I tend to think defensively -- "what happens if the trade goes against my directional bias?", assuming I have one. If I have a bullish bias and want to put on a VERTICAL spread, in my mind I would rather be short an ITM call than an OTM put, as the former has a significant delta advantage when I'm wrong, and both have similar gamma/vega/theta exposure. (I should point out that I have no appetite for naked short options, and that I also understand the bid/ask spread issues with debit spreads.) So, constructing a bull vertical with calls at least postpones the decision whether to (a) close out a losing spread or (b) merely close out the profitable short strike (should I think a rebound is imminent).
Does that make sense?
From there the question becomes whether or not I want to flip vega in a directional trade, and then for NON-directional trades is there an inherent advantage for the defensive trader in constructing delta-neutral trades using verticals, diagonals, or combinations thereof.