You asked why would anyone get a compact double? I am not putting down triples by any means, but I have compact doubles on both of my bikes. I switched from triples because they are mechanically simpler, which means they are easier to keep in tune. They are also lighter. In my case I had to switch because I needed a custom crank set to correct a 3 cm leg length discrepancy, and it was easier to build a double than triple to the specs I needed to correct my biomechanical problem. On my go fast bike I paired my 50/34 compact double with a 12/27 casette for a range of ~32-110, and on my touring bike I paired it with a 11/34 casette for a range of ~26-117, which both work well for me.