Both metrics have their uses. Personally I'm not all that interested in playing the latest hit AAA games. I am much more likely to play indie games or older games, which probably means I play more titles but the titles I play tend not to have much market share (individually or even in aggregate). You could say I'm an outlier but I feel like a lot of Linux nerds probably have similar gaming habits.

So for me it's better than 90% of all games are playable on Linux, than if a handful of games accounting for 90% of market share were playable.