I have AA, CX and BA emerald cards... but actually 2 is already enough, and I am thinking of dropping BA one next year...

Since I'm based in HK and fly CX quite a lot, CX program is great in that its the only one you can use miles to upgrade on CX... other programs can only redeem awards for flights, but no upgrade.. Also, as others have mentioned already, the ability to purchase at any published fare even when the flight is full is a big benefit... I also tend to put my CX number onto my CX flights for better recognition and service, and a much better chance of op-up.

AA is great for earning miles on other OW carriers especially with 100% bonus miles, plus lots of bonus mile promotions for domestic US segments... Plus there's 1mm and 2mm levels with give you lifetime ruby or saphire in OW...

BA is really only useful if you fly a lot on BA and need to get things like waitlists cleared or preassigned seating.. otherwise not really useful... I just got it last year coz I took a lot of BA flights and I was able to have the miles credited to Asiamiles while still getting tierpoints...