Well for pvp and fast targets maybe Emerald is better since they die pretty fast when critting
Actually, aside from Sharpshooter DHs, emeralds are not great for PvP because you don't have the luxury of standing in one spot all day to proc your crits. You have to make every hit count, so it's better to use a ruby to deal consistent damage over time.
But for general farming where you get to sustain DPS, the damage averages out for ruby and emerald.
or u can be a barb with 63 base cc, + wrath+op+br = 86% cc with weapon throw. u only need 1-3 attacks to kill someone with a wombo crit
Simply put, the marquise gems are designed to most improve the very elite of gear. To make the best just a little more "best." Because for the vast majority of players, you would net better gains from an entirely different weapon upgrade as opposed to the gem upgrade. Again, you'll see an ebb and flow for that as well, depending on the market.
As a 250k dps monk I can say that a 20 million gem giving a 3k damage gain, is more bang for the buck than a 1 billion Mempo giving a 4,5k dps gain. So at some point, when your gear is pretty great and your upgrades are pretty damn expensive, it can be a sweet little upgrade.