As soon as SVC and hrc969 found out that the alternate forms actually exist in Diamond and Pearl's .pkm file, it became clear that the forms could in theory be used against Diamond and Pearl players. Still, I am slightly surprised by the report about the Battle Recorder because of the following statement:
PokéBeach said:
- I setup a trade between Platinum and Pearl. I wanted to trade my Giratina Origin Forme with its Platinum Orb to Pearl. When entering the trading room, Giratina Origin Forme reverts back to Giratina and its Platinum Orb goes back into the backpack. When you leave the room, you have to put the Orb back on yourself to transform it into its Origin Forme.
- The same thing happens with a local battle. Giratina reverts to its regular form and the Platinum Orb goes into your backpack. It does not seem like any of the new forms can be traded or battled with while in their forms. They are exclusive to Platinum for this generation it seems.
As far as I know, PokéBeach's webmaster owns a legitimate copy of the game. It could very well be that the person whose data was watched on the Battle Recorder was doing similar experiments to those done by SVC, hrc969 and Arael; after all, the Battle Recorder doesn't reject data submitted by hackers.
But either way, the reason for incompatibility likely has to do more with the distinction between trading and battling than it does with the distinction between Diamond/Pearl and Platinum. The Club Colosseum only offers battling; the Union Room and Wi-Fi Club offer both trading and battling. That makes matters more complicated: Were it possible to enter the Union Room/Wi-Fi Club with the forms unchanged, the need to revert them would be dependent upon the player's actions in the room.
The reason that trading is off-limits is not solely because of compatibility issues with Diamond and Pearl: Game Freak clearly wanted to restrict the number of Origin Forme and appliance-based Rotom to one of each per a game. Why is it so vital? I couldn't answer that as I fail to see what makes those Pokémon more special than Deoxys and Shaymin (the latter of which has been impacted by this, though). But this is also consistent with the odd move of making Rotom's alternate forms event-exclusive and rendering them ineligible in official tournaments. It is all unnecessary hype, but sadly, it might just have worked out in the relevant companies' favor.
Finally, it could be argued that Game Freak should have programmed the forms to revert
within the trading room right before being traded. The trading process would have appeared to be like that of Deoxys in the GBA games, but in actuality, it would have been different in that Deoxys did not have to revert since its data was interpreted differently between the games, and no game actually had data for more than one form. Had this been done, there would have been no need to distinguish Union Room and Wi-Fi Club from Club Colosseum.
Unfortunately, the lack of discussion on the subject outside this forum seems to indicate that there won't be enough fan backlash to change Game Freak's mind.