>will you lose out by only checking the clipboard when the user hits your text input field? Is it that much to ask that you find the least offensive way to serve your user?

In your toy example of my app's main screen being a text box where the user can insert a tracking code, yes, I do lose by making the user wonder every time "you know I have a tracking code why are you making me type it in?" In a more realistic example of, say, Amazon, where the "track my previous order" button is not the main screen of the app, the convenience is further increased by doing the detection automatically.

And what about non-text clipboard contents? Not every interface is a text-style document into which content can be embedded. Even "Copy URL" requires knowing that "share website" shared a URL and not a website. It doesn't make sense for images at all.