Since, at least for now, DL and AA flights can be credited to AS and, in fact, AS elites are eligible for low priority upgrades on DL, there probably isn't much rationale to make AA or DL your secondary program unless you fly enough to be able to get top tier status on one of these carriers. So I would think about whether a secondary program in *A would make sense. IMO the basic choices would be UA for Y+, domestic upgrades, and SWUs (if you can get high enough status), plus access to *A award tickets with UA miles, or--especially if you can't get the 50,000 status miles and $5000 spend required for Gold on UA but nevertheless want some *A benefits--credit enough to Aegean to get their *Gold at 19,000 status miles, which would allow you access to UA lounges even on domestic *A itineraries but not give you Y+ or upgrades on UA, etc.