No.
It is another medium for sharing ones work(flickr, 500px, photobucket, facebook, etc, etc etc,).
The user defines what the site is used for, not the ones who created it.
Sort of agree and sort of disagree. If that was purely true, then you'd be allowed to upload to it from something other than a smartphone. As I understand it, the only way to get shots to it from something other than a cellphone now is to save those pictures to your phone. This is done specifically on purpose to maintain the 'purity of the medium' and encourage users to only use their cellphones. Plus, the community around the site highly discourages using non smartphone photos.
Sure, you can post photos taken on any camera, but the other poster asked if it was considered cheating, and by and large the community that creates the norms for instagram does consider it cheating. Whether you care what other people think is up to you entirely, obviously. But cheating is a concept that only makes sense when you consider other people in a community, thus the user his or herself can't define what cheating is, only a community can.