Very nice summary!

Delta seems to work best for my kids and friends/family that don't fly much, as I'm successful in adding points to their accounts every couple months or so via a survey, promo, or similar that enables them to accumulate free or cheap miles without actually flying. I have a young daughter (under two) who already has enough miles for a free ticket, despite having never purchased one nor having spent a penny (I found some loopholes in various bonus promos and stacked 'em up for her and others).

Slightly OT, but if you look at hotel rewards programs, Priority Club fits the bill well. I've earned thousands of points annually for several friends/family accounts without stays, and especially if you redeem for a 5K Pointsavers night sometimes, it's worthwhile. Marriott and SPG, on the other hand, very rarely offer surveys or promos that will allow points to be deposited without requiring any type of spending.