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.
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