I think it makes the game more fun to slide. Who wants a combat with 2 massive units that only has 2 models fighting each other? If you know you're sliding, you already know if they charge you, all their models will hit. If you play clipping, you can very easily set it up to force your opponent to bring 1-2 cavalry models into your massive static res unit and never have a chance to win, or just to not charge you. Both options then are no fun.
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