Originally Posted by
svtmike
That's because you're looking at it from the consumer side, and not the supply side. For a supplier, the "mid-compact" allows him to sell a compact, something close to a standard double and he only has to deal with a single 110BCD crank spider.
None of this helps the consumer, but part rationalization like this is very common.
There's a big advantage to the consumer to have a single 110BCD spider; it allows you the option to run anything from 46/36, 50/34, 52/36, 53/39. to 56/42 all on one crankset.
Thus you can change to the gearing you want for different terrain just by swapping chainrings, and not having to have 2 cranks.
And with stiffer chainrings you don't need a 130bcd to run bigger chainrings. This is one of the cool things with Shimano's new 9000 group; they just have one crank, not a compact or a standard, that you can spec as you want.