Yeah, I think being able to add multiple platforms to one game could work if it was just done for these specific cases where one purchase gets you the game on multiple platforms, it could also be applied to something like Portal 2 on the PS3 (Where you also received a code for the game on Steam)
The problem is, it'd only work if everybody followed procedure exactly, but I can't see that happening, and it would likely just cause confusion for anybody who doesn't go on the forums. We'd probably see lots of pages created with multiple platforms despite each version having a separate sku, or having to buy them separately.
I thought over the idea of having multiple pages, and I think it could work. If there's separate pages for each console that it's playable on, if somebody were to buy the game on, say, their PSP... they can easily find it just by searching for that version.
The question is, if somebody only downloaded the PS3 version of a game, or perhaps only the Vita version, then even though they technically own the other versions, would it really be a part of their collection if they didn't bother to download it? It's a thin line.
I think anybody who bothered to download all of the versions would likely add all three entries.
Of course, somebody may end up just adding one of the entries... the only place where this matters is on the game page itself, showing how many people have it in their collection. To me, personally, that seems like a small concession to make.