They sort of do. FireRed, LeafGreen and Emerald introduced different Deoxys formes and Platinum had a new Giratina Forme. Completely new species might complicate link battles. Also they might not care that people can fish through the game's data for unreleased Pokemon.
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Pokemon VGC2011 NW Regional - Top 8
Pokemon VGC2011 US Nationals - 54th place