I've been riding a 50/36 with a 9-speed 11-23 cassette for years. It's just seems too darn easy to spin out a 34t.

Anyway, I've always maintained the principle advantage is better front shifting for spinners like myself that would otherwise be stuck with a triple. They can also be used to provide lower, near triple chainring climbing gears with better front shifting at the expense of wider gear spacing.

If I really, really needed the lower climbing gears, I'd still go for a triple with an 12-23 cassette over a compact double with something like a 12-28. I simply find it much easier to maintain my rather high cadence while climbing with a 36t inner ring. A 39t ring either leaves me without a 14t cog in the back and a big jump between the 13t and 15t cogs or spinning out a 12t cog too easily on the steep descents that are so prevalent out here.