I had to delete the posts for being off topic, but I'd like to argue (healthily) with you on it :)
Though the birds were included in Platinum, you can't call their inclusion in HGSS "unnecessary". It's not like they were necessary at all in RGBY. There's no real backstory behind them, at least none that come to mind. They're legends for the sake of being legends.
Now, something I always thought- everyone says HGSS and DPPt take place at the same time. I now disagree- I believe HGSS takes place shortly before DPPt. Lookat the start of DP, they say the exploration team couldn't find the Red Gyarados anywhere- this is most likely because Ethan/Lyra had already captured/defeated the Red Gyarados. That's just one example. The fact that both Frontiers are the same in HGSS and Platinum, would mean that there has to be time difference somewhere, as it'd be impossible to have the Brains in 2 regions at the same time.
This leads on to the actual point you made- that there was no need for the Legendary Birds to be in HGSS, because they were in Platinum. A simple explanation for this, in light of my previous paragraph, is that Red never caught or battled the Legendary Birds when he travelled through Kanto, and neither did Ethan/Lyra three years later. Thus, the Birds flew to Sinnoh. That's my theory, anyway.