DLC gems are no different than DLC characters.
Yes they are. If I don't buy Shuma-Gorath in MvC3 the only change is that I can't use him. Gems are equipment that can be used by any character. In the worst case scenario (DLC Gems being better than others), my characters could be worse because they don't have access to the same equipment. There's a world of difference between "Kazuya sold separately" and the hypothetical "give us more money or your Kazuya sucks"--factor in that Gems might be beneficial to multiple characters and you're looking at several of your characters being inferior until you pay more money to upgrade them.
Suppose MvC3 came with only two Assists and the third was sold separately. You can still use all the characters and have some options with them, but some abilities are locked from you. People would scream bloody murder over that, especially if those abilities turned out to be the best option for some characters.
Hopefully not that bad, but "Worst Case"...
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What you are experiencing is a temporary distortion of reality...