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I don't even use gamemodes. I can't speak for others, but the reason I don't use them is that, while a gamemode with everything set up is nice, I like to tweak things, add a few extra add-ons, remove a few, etc. Gamemodes do not allow this. So... I just stick with custom.
Also as far as creating gamemodes goes, I don't even know what you can do with one. Like, all of the options available. I know there was a short tutorial up at one point. But I can't seem to find it any more.