I don't think instagram is about the limitations, but about the community. It is pretty amazing, and explains how it has gotten so huge in so short a time. there is something about it that just works. I gave up on 500px and flickr in short order because I didn't feel connected to it. I also prefer viewing photos on my phone. Much more convenient than sitting in front of my PC.
Instagram is also about the filters, and how to use them to affect the mood of the photo (of course a poorly composed photo cannot be helped by filters). I've messed with all of my DSLR photos within instagram before posting. Filtering, tilt-shifting, and square cropping have been so much fun and easy in instagram.
If you take your limitations to heart, then the Olloclip should not be used either. The 4s camera is quite a bit better than the 4, should that be limited too? I don't think the developers had any limitations in mind otherwise they would have limited posting to only images captured within the app. Seems kinda artificial to say photos must be limited to so and so equipment. That works for challenges, but not the site as a whole. All that being said I do think it is important to tag photos as to how they were captured.
I found myself in tune with the article I posted, but was taken aback when I started reading the comments. There definitely is a part of the community that feels strongly against anything but iphone captures.