Here's my opinion, it may not match others'.

The shift ratio of the rings on a cross crank (46/36=1.2777) are similar to the ratios on a road crank (52/39=1.333) or the top two rings of a MTB crank (42/32=1.313). This ratio of about 1.3 gives nice double shifting with a wide ratio cassette sequence with about 1.15 ratio per shift. 8x11-28 or -30 or -32 are good examples. This still works with a higher number of speeds, because for a wide cassette range like this, most of the cassette is still the same, and they just pack in an extra intermediate cog at the top end you don't use much, or add one more that's 4 teeth bigger on the bottom. The overall ratio gets lower from road to cross to mtb due to probable riding conditions for these bikes.

However the overall range is still not very high. It basically gets you two more ratios compared to a single. In order to maintain the same double-shift-ratio philosophy and get lower gears you need a road triple (52-39-30). In order to get a really huge range, like on a MTB, you keep the ratio for the outer rings and make the inner ring a much bigger jump. The huge downshift is ok because you use it when you really want a big bailout downshift. If you ignore the double-shift philosophy in favor of sticking with a double, you go compact, and the front downshift is huge. It's more like a low and high range. With a compact, you can get a wider gear range by sticking with the original cassette; or if you want to keep the same overall ratio as the double with a compact you use a narrower cassette and it gets you more useful ratios.