They're a good idea, it's a great marketing stratergy "Look, Yugi has one of these cards in his deck and he always wins, you need one too! Buy buy buy!" and occasionally duelists might see a good idea on there and it'll set off world wide trends. And what could possibly be a better idea than an anime you can go off and recreate yourself, ok so they're not the best genre in the world, but there's only so many times you can try kage bunshin no jutsu before you realise it just ain't gonna work, but with a card game kids can make decks like thier favourite characters and play the anime.
Personally I find them a bit samey, Yu Gi Oh at least, I haven't watched DM enough to know, but it always goes: good guy in a duel, good guy is getting beaten, good guy conveniently draws the one card they need at that exact moment and wins.