I've been doing a bit of research into the same question, as part of my planning out my next bike. I'm looking at getting a cross bike to use as a "jack of all trades commuter/century/group ride bike". Looking at my current gearing (48/38/28 11-32 8sp) and what I use/don't use and planning it around a compact double with a 9sp.

Currently, in addition to almost never using the 48/11 and 48/13 (and those only on downhills where I could otherwise rest), I find the spacing too wide, with 15%+ jumps between cogs. Also I know that I can climb hills over 9% in my 38/32 (mechanical issues prevented my from shifting to granny on a group ride with a documented climb).

This gave me an idea that I need gear-inches from ~95 at the high end to 31 at the low end, but with tighter spacing in between. A bit of playing with a gear calculator found a combo that would give me what I want 46/34 with 13-30 9sp. The 13-30 is a custom cassette that Harris Cyclery calls the "Century Special" and claims with work with even short cage derailleurs. This will give me a range of 94.7 to 30.3 (staying with my 700x28 tires) and spacing well under 13% on most shifts.

To finish off my statement, I'll quote Sheldon Brown on the ratings of capacity of rear derailleurs.

Rated maximum rear sprocket size, however, is also commonly much lower than what actually works. For instance, Shimano's models designated as "road" derailers are generally listed for a "maximum" sprocket of 27 teeth...because 27 teeth is the largest size that they make in a designated "road" cassette. However, in almost all cases, these derailers, even the short-cage models, will handle rear sprockets as large as 30 teeth in practice. (This somewhat depends on the design of the frame's derailer hanger, so once in a while you will find a particular installation where you can't use a 30, but I've never seen one where a 28 wouldn't work