So, let me say it again, who cares about the official Instagram app?
If you have a vested interest in the Windows Phone platform growing, YOU should care. Anyone who wants to see WP succeed should care; they should download, login and use the Instagram app, even it if's just to do it and then immediately switch back to 6tag. Or never use Instagram again, whatever.
WP needs big apps. Big app developers need to see that there's a demand for big apps on WP. "This third party app is better, I don't care about the official app" is not going to show them the demand. Downloading, installing, and using the official app will.
Rudy's app is great and it filled a void for sure. But if no one touches the official app because of it, I predict it will have a negative effect on the platform in the long run.
I'm not saying don't use 6tag. I'm just saying use the official app once or twice to prove to big app developers that they should be on Windows Phone as well as iOS and Android.
(And the same goes for Vine, and Snapchat if they ever put out an official app.)