I'm not sure why this is an issue at all. The same surface you use to enter your password you also use to interact with the device. So unless your interaction consists solely of unlocking the phone, then putting it away again, the screen is going to be absolutely covered with fingerprints and smudges and smears and there won't be any way to tell which ones are from the password and which are from actual usage.

I just took a look at my own iPhone, and it bears this out. On the bottom half of the screen, there are a series of fingerprints and a giant smudge. If you were to try and guess my password from the clear prints, you'd end up pressing the wrong digits entirely.